340 



TUNICATA. 



The third plane of cleavage is equatorial and, in the eight-celled 

 stage that follows (Fig. 150 B), separates four smaller cleavage-spheres 

 from four larger. According to all observers, the later ventral half 



of the body is thus divided from 

 the dorsal half. The four smaller 

 spheres (a) which lie near the 

 animal pole, and are said to 

 represent the ventral surface of 

 the body, are purely ectodermal 

 in character, while the four larger 

 blastomeres (b) which belong to 

 the vegetative half (the future 

 dorsal half) are said by van 

 Beneden and Julin to show a 

 mixed character. They give rise, 

 through division, to the large 

 entoderm-cells, smaller ectoderm- 

 elements being simultaneously 

 abstricted from them, these latter 

 then joining with those of the 

 ectodermal half of the body. 

 According to Seeliger and 

 Davidoff, these cells are,- on the 

 contrary, purely entodermal.* 



Even at this stage, certain 

 displacements of the blastomeres 

 can be observed, and these inter- 

 fere with the regularity of the 

 later course of the cleavage. 

 This regularity is also disturbed 

 by the fact that the ectoderm- 

 cells, from this time onwards, 

 divide more rapidly than the 

 entodermal elements. It is, 

 however, possible to distinguish 

 a sixteen-cell stage brought 

 about by meiidionial furrows, a 

 thirty-two-cell stage caused by 

 equatorial division, and a later 



* [According to both Castle (Nos. II. and III.) and Samassa (No. XXXII.), 

 van Beneden and Julin, together with Seeliger, were mistaken in their 



FlG. I'd. — Gastrulation in Olavelina (after 

 Sekuger). .1, placula-stage ; B, the 

 invagination commencing ; C, alter 

 complete invagination (median .section). 

 ec, ectoderm ; en, entoderm ;/, cleavage- 

 cavity. 



