96 



TUNICATA. 



The generative organs are always united in the same animal. 

 The formation of villi on the surface of the egg-membrane by the 

 follicular cells surrounding the ovum is remarkable. The origin of 



U 



FIG. 564. Development of Phallusia mammitlata (after Kowalevski). a, B!as-tosphere beginning 

 to invaginate ; Fh, segmentation cavity, b, Gastrula with blastopore (0) ; Ed, endoderm ; 

 Ch, commencing uotocbord (urocliord). c, Later stage. Ek, Ectoderm ; N, rudiment of the 

 still open neural canal, d, Stage with body and tail; Ed', endodermal layer in the tail; 

 M, muscular cells in tail, e, Just hatched larva ; Rg, antej-ior swelling of the spinal division 

 of the neural tube ; Rm, posterior part of neural tube ; Gb, dilated anterior part of neural 

 tube (cerebral vesicle), with otolith projecting into it; F, opening of Gb ; A, eye; 0, in- 

 vagination of mouth ; Ph, pharyngeal cavity ; Ed, endostyle ; D, commencing intestine ; 

 Kl, atrial opening; Bl, blood corpuscles; Up, papilla for attachment. /, Two days' larva 

 (only the beginning of the tail is represented) ; Ks, ^Ks, branchial stigmata : Bb, branchial 

 vessel between them ; D, intestine. 



the so-called test-cells (follicle-cells which have migrated inwards) 

 over the substance of the yolk on the inside of the egg-membrane, is 

 also worthy of note. 



