THE DEVELOPMENT OP THE MOLE. 131 



(Stage g) it is wider in front of the first protovertebra than it 

 is in the latter and posterior regions^ and faint indications of a 

 division of the brain into portions may be discerned in section, 

 and to some extent also in the surface view of this stage ; the 

 hind-brain, with its somewhat thinner roof, is of considerable 

 length and blends into an anterior portion in which the roof is 

 thicker. Stage h shows some little advance upon this; the 

 cranial flexure has begun (fig. 34) and the cavity of the brain 

 has increased in size, the roof of the hind-brain also is thinner 

 and wider than before (fig. 23). 



At the close of Stage j three divisions of the brain are indi- 

 cated (fig. 49). There is a well-marked cranial flexure, and at 

 what is now the anterior end of the animal the mid-brain is 

 situated. The cavity of the mid-brain is partially separated 

 from that of the fore-brain by a constriction of the walls at the 

 junction of the two, but the structure of the wall is very 

 similar in both portions. The hind- and mid-brains pass into 

 one another without any such constriction, but the thin roof of 

 the former distinguishes it from the latter. The lower wall of 

 the hind-brain at the posterior end is now much folded. The 

 lower wall of the fore-brain is curved downwards, forming a 

 short and wide diverticulum which marks the first appearance 

 of the infundibulum. The apex of the infundibulum comes 

 into close connection with the anterior end of the alimentary 

 tract and with the notochord overlying it (fig. 49) . 



The Optic Vesicles. — The optic grooves seen in the head in 

 surface view in figs. 2 and 5 are the rudiments of the optic 

 vesicles; they are shown in section in fig. 16. Later (Stage 

 h), when the medullary groove forms a closed canal in the 

 head region, these grooves become wide lateral diverticula pro- 

 jecting from the anterior portion of the brain, and constitute 

 the optic vesicles (fig. 20). They are situated dorsally on 

 each side the middle line, and are projected outward and 

 somewhat downwards and backwards. 



Such a condition is clearly shown in surface view in fig. 9. 

 Sections of this stage show a very similar condition as regards 

 the development of the vesicles ; they merely extend slightly 



