134 EDWABD MCCRADY, JR. 



In the caudal region the neural crest is still in a primitive 

 condition; and in very early specimens there is even a pos- 

 terior neuropore in the tail. 



Within the central nervous system the ependymal, mantle, 

 and marginal layers are well differentiated in the hind-brain 

 and cervical cord (fig. 42, sections A to E). 



Larsell ('35) has found two commissures arising in the 

 cerebellum in stage 30. The cerebellar commissure, which is 

 the more anterior of the two, arises in connection with the 

 trigeminal ; and the lateral commissure, the more posterior, is 

 associated with the seventh nerve. According to his inter- 

 pretation these two primitive commissures form the basis 

 of two functionally and phylogenetically different parts of 

 the adult cerebellum the corpus cerebelli, associated primi- 

 tively with the fifth nerve, and the flocculo-nodular lobe as- 

 sociated with the seventh. 



Miscellaneous details. In the heart the interventricular 

 septum (E), the interatrial septum primum (A), and the 

 endocardial cushions in the atrio ventricular canal (fig. 42, C 

 .and D) are just beginning to form. Capillaries from the left 

 part of the atrium invade the mesoderm of the lungs. From 

 the proximal part of this plexus a single large pulmonary 

 vein is organized during stage 30. It runs between the ducts 

 of Cuvier and empties into the atrium just to the left of the 

 septum primum. Some transitory capillary twigs from the 

 anterior cardinal also ramify in the region of the trachea. 



The first and second aortic arches are reduced to capillary 

 plexuses. The third is at the height of its development. The 

 fourth and fifth are well formed (fig. 43). It is interesting 

 that the fifth is well developed and arises before the sixth 

 a primitive condition not heretofore described in a mammal. 



The lens cup pinches off to form a vesicle (fig. 42, D). The 

 optic vesicle begins to invaginate to form a cup (D). The 

 naso-oral groove deepens very markedly (D). 



The thyroid diverticulum pinches off to form a vesicle 

 situated just in front of the fork of the ventral aortae (re- 

 construction). Rathke's pouch deepens and constricts, but 

 is not yet pinched off (reconstruction). 



