What is Saccus in Embryology?
Saccus simply means sac or bag and is is closed cavity. The lymph nodes develop from the structures, see this image:

 


Superior cardinal vein (jugular vein)
Jugular lymphatic sacs
Right subclavian vein
Axillary lymphatic sacs
Left brachiocephalic vein
Thoracic duct (bilateral)
Lumbar lymphatic sacs
Iliac lymphatic sacs


The image is taken from this webpage, which should be interesting for you.