20 



PHYLUM TUNICATA (UROCHORDA). 



Development of the tadpok larva. The segmentation is 

 complete and leads to the formation of a blastosphere, from 

 which the gastrula arises by invagination. The gastrula 



FIG. 15. Development of r/nil/nxiii niniininllatii (alter Kowalevsky from Glaus), a Com- 

 mencemeut of invayination ; / h cleavage-cavity. b Gastrula with blastopore ; ch rudi- 

 ment of notochonl ; hi/ endodenn. (. Later stage ; Ek ectoderm ; ^V rudiment of neural- 

 canal ; Eil' eiulodenn of future tail region, d Stage with body and tail ; Ed' endoderm of 

 tail ; M muscular cells in tail, <> Just-hatched larva ; A eye ; Bl blood corpuscles ; _D k com- 

 mencing intestine ; !;,! endo^uie. ; F opening of cerebral vesicle into mouth ; Ob cerebral 

 vesicle with otolith projecting from its floor ; Hp papilla for attachment ; Kl one of the atrial 

 invaginations ; O mouth ; I'll pharynx ; R g anterior swelling of post-cerebral region of nerve 

 tube ; lim posterior part of nerve tube. / Two days larva, only the anterior part of the 

 tail is represented ; 1 ks, 2 ks branchial stigmata Bb blood shins between them ; D in- 

 testine. 



elongates in the future antero-posterior axis and the blastopore 

 comes to lie on the posterior end of the dorsal surface (Fig. 

 15 b). A flat median groove of ectoderm appears along the 



