castle: embryology of ciona ixtestinalis. 235 



4. The third cleavage plane is at right angles to both the preceding, 

 i. e. equatorial in position. It separates four smaller cells lying nearer 

 to the polar globules and more abundantly supplied with yolk, from four 

 larger ones more remote from the polar globules and richer in protoplasm. 



The former are destined to give rise to the dorsal or endodermal 

 hemisphere of the embryo; the latter, to the ventral or ectodermal 

 hemisphere. 



The protoplasmic accumulations mentioned under paragragh 1 always 

 fall in the posterior pair of cells of the ventral hemisphere (viz. Z)*-\ C'^-^) 

 close to its line of contact with the dorsal hemisphere. This pair of cells 

 is never in contact with the anterior pair of cells of the dorsal hemi- 

 sphere, but the anterior pair of cells of the ventral hemisphere (viz. 

 ^4.1^ ^4.1^ jg invariably in contact with the posterior pair of cells of the 

 dorsal hemisphere (viz. c*-^, d*-"^). 



The fact just stated affords a ready and unfailing means of orienting 

 the 8-cell stage. This was recognized by Chabry in the case of Asci- 

 diella, and is shown by an examination of Seeliger'^ figures to be equally 

 true for Clavelina. It pi'obably holds good among all the simple and 

 social Ascidians. 



5. The 16-cell stage is usually reached by simultaneous divisions in 

 both hemispheres. Sometimes, however, the cells of the ventral hemi- 

 sphei'e at this cleavage divide sooner than those of the dorsal hemi- 

 sphere, thus giving rise to a 12-cell stage, but this very soon changes 

 to a 16-cell stage by the cleavage of the cells of tlie dorsal hemisphere. 



6. As the egg passes from, the 16-cell stage, cleavage invariably occurs 

 earlier in the cells of the ventral hemisphere, i. e. the descendants of the 

 four larger cells of the 8-cell stage, than it does in the cells of the dorsal 

 hemisphere. A 24-cell stage results, in which the cells of the ventral 

 hemisphere, being twice as numerous as those of the dorsal hemisphere, 

 cover more surface and begin the process of overgrowth (epiboly), for- 

 cing the cells of the dorsal hemisphere into a somewhat columnar form. 

 (See Plate VII. Fig. 44.) 



B. Later Stages of Cleavage. 



(«) From 24-cell to 46-cell Stage. 



The 24-cell stage was taken as the point of departure in my prelimi- 

 nary paper ('94), and the cell lineage was traced in detail through a 

 stage of 46 cells. I shall not repeat except in the form of a brief resume 

 what was there said regarding those stages, but shall content myself 



