170 ELEMENTARY BIOLOGY. [CHAP. 



pore, and a dorsal or neural canal, at this period 

 in communication posteriorly with the archen- 

 teron and blastopore by a short narrow passage 

 (neurenteric canal). 



/?. The notochord; a median rod of slightly vacuo- 

 lated cells, lying in the interspace between the 

 archenteric and neural tubes. 



y. The neural tube; enlarged in front (brain) and 

 overhanging the notochord. 



8. The archenteron; its extent; its roof, formed by 

 the hypoblast cells; its floor, still bounded by 

 undifferentiated lower layer cells, now compara- 

 tively few in number. 



e. The mesoblast ; composed of irregular scattered 

 cells and more extensive than in the earlier 

 stages examined. 



i. At 15 — 18 days (cf. i. i); transversely oblique, to 

 pass through the visceral arches and external 

 branchiae. 



a. The brain ; a large vesicle, situated at the an- 

 terior end. 



/?. The mesenteron (archenteron) ; a large central 

 cavity behind and in a line with a, wide in front, 

 constricted behind. 



y. The visceral f ouches ; paired diverticula of the 

 hypoblast, 4 or 5 in number on either side, their 

 blind ends abutting against the epidermis (if the 

 section be much shrunk their walls may be in 

 close apposition). 



8. The visceral arches : mesoblastic aggregates 

 alternating with y; each lying behind its cor- 



