DEVELOPMENT OF THE NERVOUS SYSTEM 515 



consult figure 37 which was drawn from the same specimen. Three 

 infoldings are prominent dorsally, the cephalic fold (the velum 

 transversum) the most so. Cephalad to this is the paraphysial 

 arch which as yet is indistinct and shows no evidence of a para- 

 physial evagination. The cells of this arch, as well as those of 

 the velum, are arranged in a single or irregular double layer. 

 Posterior to the velum there is an arch limited caudally by a 

 prominent infolding in which the posterior commissure will 

 develop. At about the mid-point in this arch, there is a slight 

 projection inward, which represents the constriction which was 

 prominent in figure 29 and separated the roof of the diencephalon 

 into two segments. The first of these segments is the post-velar 

 arch and as yet it has not begun to lengthen more rapidly than 

 have the other parts of the roof. Only at this early stage (fig. 29) 

 is the roof of the diencephalon distinctly divided into two seg- 

 ments and by the next stage this dividing point will have disap- 

 peared. Caudad to the diencephalic arch lies the first mid-brain 

 neuromere. The cell arrangement of the constriction which 

 divides the mid-brain from the diencephalon is different from that 

 of the velum transversum. In the former the invagination appar- 

 ently is due to a proliferation of cells, while in the latter it is a 

 simple infolding, there being no thickening (if anything, a flatten- 

 ing) of the cells. This might lend some support to the contention 

 of Johnston ('09) that the formation of the velum is primarily 

 due to the withdrawal of material for the formation of the optic 

 vesicles. 



Just cephalad to the di-mesencephalic infolding is a dorso- 

 caudal outgrowi^h. This is the rudiment of the posterior portion 

 of the epiphysis and in this evagination a cavity will later appear. 



By Stage X (figs. 2, 38) the paraphysis has become a well 

 marked evagination composed of simple cuboidal cells. Cephalad 

 to the paraphysial out-pocketing is a second out-growth which 

 reaches its maximum development at this stage. In specimens 

 between figures 1 and 2 at this region two evaginations of about 

 equal size can be seen. The caudal one grows the more rapidly 

 and forms the paraphysis, the cephahc one becomes less distinct 

 and probably contributes to the true paraphysis. The second 



