ASCIDIACEA FORMATION OF THE GERM -LAYERS. 



343 



a 



symmetry is still more marked, the future anterior end of the body 



becoming swollen in consequence 



of the increased curvature of its 



two layers (Fig. 152 B). This 



arching is connected with the 



gradual narrowing of 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 C). Originally, the blastopore 



is a wide oval aperture, but in 



later stages it is pear-shaped, and 



it finally becomes a small posterior 



aperture (Fig. 153, b-1"). 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 Amphioxux, 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 C). 



The dorsal side is recognisable by e ' 



its natter condition, and shows, at FlG . 152. Three consecutive gastrula- 



its posterior end, the remains of 

 the blastopore (p) ', the ventral 

 side, on the contrary, is arched. 

 VAN BENEDEN and JULIN have 

 pointed out that the posterior end 



rf the body, at the gastrula-stage, 



is always marked by the presence 



of two small wedge-shaped ectoderm-cells lying at the edge of the 



commencing; B, appearance of the 

 bilateral symmetry ; C, narrowing of 

 the blastopore ; a-b, principal axis of 

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

 tudinal axis of the body ; d, dorsal 

 side; ec, ectoderm; en, entoderm;/, 



*KS* 1 "' U ^ < " P re ' "' 



