ORGANIZATION AND CELL-LINEAGE ОҒ ASCIDIAN EGG. 53 
umnar, while the dorsal cells become relatively broad and flat (text figs. ХІ, XII, 
ХІХ, ХХ), This change of shape has been observed and commented upon by 
Van Beneden and Julin, Samassa, and Castle, and both of the latter authors attri- 
bute the columnar form of the ventral cells to the pressure exerted upon them by 
the overgrowth of the cells of the dorsal hemisphere; both regard this overgrowth 
as the beginning of gastrulation (epibole). Whatever may be the cause of the 
shapes of the cells at the two poles, whether purely mechanical or not, it is certain 
that this is not the beginning of gastrulation, since, as I will show later, the 
columnar cells of this stage become the flattened ectoderm cells of later stages, 
while the flattened cells of this stage become the columnar endoderm cells of the 
gastrula. 
6. Sixth Cleavage ; 32-64 cells. (Figs. 45-45, 120-150, 194-197). 
` In this cleavage the divisions are not synchronous, the cells of the dorsal hemi- 
sphere dividing before those of the ventral as in the preceding cleavage, and some 
of the cells in the posterior half dividing later than those in the anterior one. 
Accordingly it would be possible to sub-divide the period between the 32-cell and 
64-cell stages into a 44—cell, a 46-cell and a 48-cell stage, as Castle does. These 
stages, however, are of brief duration and all the cells of the sixth generation 
divide before any of the seventh do; therefore, the sixth cleavage is distinct from 
preceding and succeeding ones. 
The spindles appear in the four chorda-neural-plate cells at the anterior border 
of the dorsal hemisphere in a nearly dorso-ventral direction. Тһе four ventral pro- 
ducts of this division (A**, A*5) form a band of small cells around the anterior bor- 
der of the egg just dorsal to the equator; these cells ultimately give rise to the 
posterior part of the neural plate; the dorsal products (A*?, А??) give rise to the 
chorda (figs. 119-125). Тһе neural plate cells are small and contain little or no 
yolk, whereas the chorda cells are larger and are yolk-laden (text figs. XIX, XXI, 
XXIII); this cleavage of these cells is therefore markedly differential. 
While these cells are dividing, all of the endoderm cells divide; these are the 
four median cells which meet at the vegetal pole (A*', Де, B*', В“) and a single 
pair of cells which lie lateral to these and in front of the transverse (second cleav- 
age) plane (A, А%) The spindles in the median cells are antero-posterior in 
direction, while those in the lateral cells are nearly transverse (figs. 120, 121, 123). 
These divisions are equal and non-differential in the median cells; in the latara 
cells the division is differential, the inner product (A*5) being rich in yolk, the 
outer (А?) containing more protoplasm; the former is an endoderm cell, the latter, 
according to Castle, mesenchyme. Ву these divisions ten endoderm cells are pro- 
duced, five on each side of the mid-line, and two mesenchyme cells (figs. 44-46). 
While the divisions of the endoderm and chorda-neural-plate cells are occur- 
| . ring, the most anterior mesoderm cell (B*?, В°?), forming the point of the crescent 
` on each side, divides, the spindle lying in a nearly dorso-ventral direction (figs. 43, 
52 44, 120-129, 193, 194). "This division, in fact, sometimes slightly precedes that of 
