DOLIOLUM — CLEAVAGE. 



383 



has been called the vitelline membrane by Uljanin, and for a long 

 time shows, on the external surface, remains of the original follicular 

 epithelium (/). Since, however, both Gkobben and Fol (No. 21) 

 observed within this membrane cells which without doubt correspond 

 to the test-cells of the Ascidiacea we may perhaps rather regard it 

 as the equivalent of the chorion of the Ascidian egg. 



ms 



Fig. 178.— Three embryonic stages of Doliolwm MuUeri (after Uljanin). A, gastrula- 

 stage ; B and C, embryos at later stages of development, ch, chorda ; ec, ectoderm ; 

 /, follicle-cells ; m, egg-shell (chorion) ; ms, mesoderm ; n, rudiment of the nervous 

 system. 



Cleavage is total and almost equal, and closely resembles that 

 described above in connection with the solitary Ascidians. The 

 blastula and invagination-gastrula (Fig. 178 .4) observed by Uljanin 

 were comparable to the similar stages in the Ascidiacea. 



We have very incomplete accounts of the subsequent stages. At 

 the next stage observed by Uljanin and depicted in Fig. 178 B, 

 the embryo is pear-shaped, and within it we can see three distinct 

 rudiments. A large dorsal cell-mass (n) is regarded by Uljanin as 



