CASTLE: EMBRYOLOGY OF CIONA INTESTINALIS. 235 
4. Tho 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 aceumulations mentioned under paragragh 1 always 
fall in the posterior pair of cells of the ventral hemisphere (viz. D**, 041) 
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. 
At, B^) is 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’s figures to be equally 
true for Clavelina. It probably 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- 
sphere 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 the 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 VIT. Fig. 44.) 
B. Laver STAGES OF CLEAVAGE. 
(a) From 24-cell to 46-cell Stage. 
Tho 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 résumé 
what was there said regarding those stages, but shall content myself 
