CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a National Phase Entry of International Application No. PCT/EP2011/058452, filed on May 24, 2011, which claims priority to French Patent Application Serial No. 1054959, filed on Jun. 22, 2010, both of which are incorporated by reference herein. 
     BACKGROUND AND SUMMARY 
     The present invention relates to a Petri dish for cultivating micro-organisms, which consists of two members, i.e. a receptacle and an additional lid and which both have a revolution shape. 
     Such Petri dishes are widely used notably in laboratories and these dishes are provided in order to be superposed so as to form stacks. This notably allows the operators to form different batches of Petri dishes, for example so as to sort them out according to the nature of the culture which is made therein. Documents JP-2003 047459, JP-2003 102463, US-2002/045245 and U.S. Pat. No. 4,160,700 illustrate the state of the art in this matter. However, if this dish stacking function is possible, to this day, there does not exist any means which allows retention of the dishes attached to each other. 
     Also, up to now, the handling of culture dishes requires the use of transport containers and of external systems for attaching the dishes together. Thus, it is common that the users make use of an elastic or of adhesive tape for securing the dishes together. It is understood that the use of external means involves control of their sanitary quality, and therefore of their sterilization. In certain cases, these constraints for handling and transporting Petri dishes are solved by using an additional disposable transport tool, thereby allowing the displacement of a batch of dishes from one place to another, which in all the scenarios involves an additional cost. 
     The present invention has the object of solving these difficulties. For this purpose, it proposes a Petri dish which consists of two members, i.e. a receptacle and an additional lid, which both have an axisymmetrical shape (revolution shape) and which are each delimited by a bottom wall and at least one peripheral wall, that said receptacle and lid bearing additional locking means laid out so that, upon forming a stack formed with the superposition of at least two dishes, the locking means borne by a first member of a first dish may cooperate with locking means of the second member of the second dish, making them thus interdependent on each other, one of said members including a peripheral wall which extends above and below the bottom wall, while the other one is dimensioned so that, upon forming said stack, said member of a first dish is partly engaged into the other member of a second dish, in a housing delimited by the bottom wall and a portion of said wall, characterized by the fact that said additional locking means are able to cooperate by a relative rotary movement of one dish relatively to the other, that said locking means are borne by the peripheral walls and that they consist in protrusions. Thus, according to the invention, the members which make up each of the Petri dishes integrate within themselves means for locking to another dish. By their presence, it is therefore possible to form stacks of dishes, in which each dish is made interdependent on its neighbor(s) so as to form an assembly which may be handled without any other external connection means. 
     According to advantageous and non-limiting characteristics of this dish:
         said protrusions are at least two in number, are angularly equidistant and extend over an angular fraction of said wall;   said protrusions have the shape of portions of mating (complementary) helicoidal ribs;   the bottom wall of one of said members bulges outwards and is elastically deformable so that it occupies a substantially planar position. Other characteristics and advantages of the present invention will become apparent upon reading the detailed description which follows, of a preferential embodiment.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       This description will be made with reference to the appended drawings wherein: 
         FIG. 1  is a very schematic view, according to a diametrical sectional view, of a Petri dish structure according to the invention; 
         FIG. 2  is a side view of the lid of such a Petri dish, while  FIG. 3  is an enlarged view of the area marked by a circle in the previous figure; 
         FIG. 4  is a top view of the lid of  FIG. 4 , while  FIG. 5  is an enlarged view of the area marked with a circle in the previous figure; 
         FIG. 6  is a bottom view of a receptacle of a Petri dish according to the invention, while  FIG. 7  is an enlarged view of the area marked by a circle in the previous figure; 
         FIG. 8  is a perspective view of the receptacle of  FIG. 6  while  FIG. 9  is an enlarged view of the area marked by a circle in the previous figure; 
         FIGS. 10 ,  12 ,  15  and  18  are cross-sectional views of the receptacle of a dish B 1  and of the lid of a dish B 2 , in different positions which finally lead to their being interdependent on each other; and 
         FIGS. 11 ,  13 ,  14 ,  16 ,  17  and  19  are detailed views of the areas marked by a circle in the figures located just above. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention most particularly applies to a Petri dish such as the one which is very schematically illustrated in  FIG. 1 . Like all Petri dishes, the latter consists of two members, i.e. a receptacle  2  and an additional lid  3  and both have a revolution shape. They are for example made in transparent plastic material such as crystalline polystyrene. 
     The receptacle  2  includes a generally planar bottom wall  20  which is surrounded by a peripheral wall  21 . As shown in this figure, this wall extends both above and below the plane in which the bottom wall  20  is contained, in order to form an upper wall portion  210  and a lower wall portion  211 . In the embodiment shown, the bottom wall  20  also includes another wall  22 , a so called “inner wall”, so that there exists an annular space between both aforementioned coaxial walls. The bottom wall  20  delimits with the inner wall  22  the space in which a culture medium is deposited. 
     The lid  3  consists of a bottom wall  30  and of a peripheral wall  31 . The latter is dimensioned so as to be able to be positioned against the bottom wall  20  of the receptacle  2 , between the walls  21  and  22 . These receptacle  2  and lid  3  may include means not shown which allow them to be immobilized relatively to each other. 
     Under the letter L is referenced in this figure, an open housing which is delimited by the bottom wall  20  of the receptacle and the lower portion  211  of the wall  20 . This housing has a diameter D. The lid  3  is as for it dimensioned so as to have a diameter d such that this lid may be inserted into the annular space located between the walls  21  and  22 , as already mentioned, but in such a way that the lid of another identical dish may be inserted into the aforementioned housing L. 
     This is the situation shown in  FIG. 1  wherein the lid of the second dish is illustrated as a profile in dashed lines. This dish configuration is a traditional configuration. However, in certain embodiments not shown the structure of the receptacle  2  and of the lid  3  may be inverted. In other words, by flipping  FIG. 1  by 180°, the lid referenced as  3  would be formed by the receptacle  2  and vice versa. 
     In the embodiment of  FIGS. 2  and the following figures, the lid  3  has on its peripheral wall  31 , protrusions or nipples referenced as  4 . One is dealing here with a lid which includes three identical and angularly equidistant protrusions  4 , as this is well visible in  FIG. 4 . These protrusions extend radially outwards while originating on the external face of the wall  31 . They have two upper and lower faces  41  parallel and tilted by an acute angle a relatively to the horizontal. Both of their vertical lateral faces are referenced as  40 , and their end face as  42 . 
     Now referring to  FIGS. 6 to 9 , it is seen that the receptacle  2  also includes the same number of additional radial protrusions  5 , which are borne by the lower portion  211  of the wall  2  and turned towards the inside of the dish. Seen from the front, these protrusions assume an L-shape, with a base  51 , the upper face  510  of which is extending upwards, and with a vertical branch  52 , the face  520  of which forms an abutment. The transverse extension of these protrusions is provided so that, if a receptacle  2  and a lid  3  are superposed, the additional protrusions  4  and  5  interfere when they are brought closer to each other. In some way, the protrusions  4  and  5  which have just been shown, have the shape of a portion of complementary helicoidal ribs. 
     In  FIGS. 10 and 12 , the receptacle  2  of a first Petri dish B 1  as well as the lid  3  of a second Petri dish B 2  are illustrated. These figures show the relative positions of both of these members, when it is desired to make them interdependent on each other. Of course, in practice, the receptacle  2  of the dish B 1  is covered with its lid  3 , while the lid  3  of the dish B 2  is assembled to its receptacle  2 . The latter however has not been illustrated in the figures in order not to unnecessarily burden them. 
     It will be noted, with reference to  FIG. 10 , that the bottom wall  30  of the lid  3  slightly bulges, with a convexity turned towards the outside of said lid, this bottom wall having the capability of being elastically deformable when pressure is exerted on said wall. In other words, subject to such pressure, it has the capability of being brought back into a position where it is substantially planar. It should be noted that in  FIG. 10  and the following figures, the protrusions  4  which equip the lid  3  are seen in transparency, which explains that they have an inverted orientation with respect to that of  FIGS. 2 and 3 , in which they are seen from the outside of said lid. 
     With reference to  FIGS. 10 and 11  and with view to forming a stack of dishes, two dishes B 1 and B 2  are brought closer together so that the lid  3  of the lower dish B 2  partly enters the housing L of the receptacle  2  of the upper dish B 1  and the protrusions  4  and  5  do not coincide vertically (in other words they are not found vertically below each other). This is illustrated in  FIG. 10 . This closing-in movement continues until the bottom wall  30  of the lid  3  abuts against the bottom wall  20  of the receptacle  2 . In this position, the lid  2  has a rated height e 1  and which corresponds to the cumulated height of the wall  31  and of the bottom wall  30 , because of its slightly curved shape. 
     Referring to  FIGS. 13 and 14 , it is then seen that the face  510  of the protrusion  5  is at a slightly upper level than that of the lower face  41  of the protrusion  4  so that even if a rotary movement is imparted to one of the dishes, relatively to each other, said protrusions cannot cooperate but on the contrary will abut against each other. On the other hand, and as illustrated in  FIG. 15  by applying pressure on the upper dish B 1 , the deformation of the bottom wall  30  of the lid  3  is made so that the latter from now on occupies a rated height e 2  of less than the height e 1 . By doing this, said faces  510  and  41  are at the same level, so that by a rotary movement of a dish relatively to the other, it is possible to engage the protrusion  4  into the protrusion  5 . 
     The tilted orientation of the faces  41  and  510 , in the fashion of portions of helicoidal ribs, facilitates this engagement. Of course, one skilled in the art will be able to adapt the dimensions and the relatively positions of the protrusions  4  and  5  so that they cooperate intimately. 
     Finally, the situation of  FIG. 18  is then attained in which both protrusions are engaged into each other. It will be noted that the vertical branch of the protrusion  5  forms an abutment which prevents any continuation of the rotary movement of one dish relatively to the other, so that the protrusions are locked in this condition. By exerting traction on one dish relatively to the other, in a direction generally perpendicular to their bottom wall, it is absolutely impossible to detach them from each other. Such a detachment can only be applied voluntarily, by performing a rotary movement in the opposite direction to the one which was performed earlier. By making use of the means described above, it is therefore possible to form stacks of Petri dishes which are made interdependent on each other, two by two, so that it is possible to envision the lifting of such a stack simply by grasping the upper dish of the stack. 
     Of course, the present invention applies to all the structures of Petri dishes which include additional locking means borne by the members formed by the receptacle and the lid. In the example described above, the lower portion  211  of the wall  21  of the receptacle  2  is intended to be continuous. However, nothing prevents certain areas from having discontinuities notably in order to lighten said Petri dish at the most.
A Petri dish has members including a receptacle and an additional lid, both having an axisymmetric shape and which are delimited by a bottom wall and at least one peripheral wall, the receptacle and lid bearing complementary lock sections so that when a stack is formed by superposing at least two dishes, borne by a first component of a first dish which can engage with the lock section of the second component of the second dish, thus making them rigidly connected to one another. These lock sections cooperate by relative rotational movement of one dish with respect to the other, the lock sections being borne by the peripheral walls and including protrusions.