170 



ELEMENTARY BIOLOGY. 



[CHA] 



pore, and a dorsal or neural canal^ at this perio( 

 in communication posteriorly with the archei 

 teron and blastopore by a short narrow passage 

 {neurenteric canal). 



p. The notochord; a median rod of slightly vacuc 

 lated cells, lying in the interspace between th^ 

 archenteric and neural tubes. 



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



overhanging the notochord. 

 S. The archenteron ; its extent; its roof^ formed bj 



the hypoblast cells; its floor, still bounded b] 



undifferentiated lower layer cells, now compara^ 



tively few in number. 



€. The 77iesoblast ; composed of irregular scattere( 

 cells and more extensive than in the earliei 

 stages examined. 



At 15 — 18 days (cf. i. i)\ transversely oblique, t( 

 pass through the visceral arches and externaj 

 branchiae. 



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

 terior end. 



p. The mesenteron (archenteron) ; a large central 

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

 constricted behind. 



y. The visceral pouches ; paired diverticula of th( 

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

 blind ends abutting against the epidermis (if the 

 section be much shrunk their walls may be ii 

 close apposition). 



8. The visceral arches: mesoblastic aggregates 

 alternating with y; each lying behind its cor-| 



