W IIUAi K\ -FORMATION OF TFIF GERM- \, A VERS. 



343 



.-/ 



ec . 



-en — L_w 



symmetry is still more marked, the future anterior end <>t' the body 

 becoming swollen in consequence 

 of the increased curvature of its 

 two layers (Fig. 152 B). This 

 arching is connected with the 

 gradual narrowimjof the blastopore 

 which takes place on the dorsal 

 side of the embryo in such a way 

 that its last vestige lies near the 

 posterior end of the body (Fig. 

 153 ('). ( friginally, the blastopore 

 is a wide oval aperture, but in 

 later stages it is pear-shaped, and 

 it filially becomes a small posterior 

 aperture (Fig. 153, b-b"). This 

 narrowing of the blastopore is 

 caused principally by the inward 

 growth of its anterior and lateral 

 margins, the posterior edge re- 

 maining unchanged. We have 

 here conditions exactly similar to 

 those in Amphioxus, and we may 

 assume a continuous closure from 

 before backward of the blastopore 

 which originally extended along 

 the whole length of the dorsal 

 surface. 



During these stages even, the 

 embryo becomes somewhat elon- 

 gated in the direction of the 

 longitudinal axis (Fig. 152 G). 

 The dorsal side is recognisable by 

 its flatter condition, and shows, at 

 its posterior end, the remains of 

 the blastopore (j>) ; the ventral 

 side, on the contrary, is arched. 

 vax Bf.neden and Julin have 

 pointed out that the posterior end 

 of the body, at the gastrula-stage, 

 is always marked by the presence 

 of two small wedge-shaped ectoderm-cells lying ;it the edge of the 



Fig. 152. Three consecutive gastrula- 

 stages of Phalhisia maininillata (alter 

 Kowai.evsky). .1. the invagination 

 commencing ; B, appearance of the 

 bilateral symmetry; C, narrowing of 

 the blastopore : a-b, principal axis oi 

 tin- gastrula-stage; c-c', later longi- 

 tudinal axis nt the body ; d, 'I' 

 side; ec, ectoderm; en, entoderm ;f, 

 cleavage-cavity ; /<. blastopore ; <\ 

 ventral side. 



