ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 65 
The second factor of the gastrulation, zzz., the overgrowth of the cells sur- 
rounding the endodermal area has been well described by Castle. It is the result 
in part of the first factor and also of the more rapid division of the cells of the ecto- 
derm and the corresponding retardation of division in the endodermal cells. "This 
overgrowth occurs in the anterior quadrants from in front, the chorda cells over- 
growing the endoderm and the neural plate cells the chorda; in the posterior quad- 
rants it occurs from the sides, the muscle cells overgrowing the mesenchyme, and 
the ectoderm the muscle cells. At three points this overgrowth is long delayed, at 
the posterior pole where there is a deep notch in the blastoporie rim which persists 
until the blastopore has nearly closed, and at the right and left sides of the blasto- 
pore where the overgrowth is slow. This leads to the peculiar form of blastopore, 
wide in front and narrow behind, which is found among ascidians. 
8. Eighth Cleavage; 112-132 cells, 132—218 cells. (Figs. 155-147, 205). 
The eighth cleavage first appears in the two anterior muscle cells of each side 
(B**, B*?), the spindles being nearly transverse to the antero-posterior axis of the 
embryo (figs. 135, 156). This division is equal and non-differential, and there result 
four muscle cells on each side, an anterior pair (B*™, 13916) and a posterior pair (B*™, 
B*™*). When first formed the median cells of each of these pairs lie at a higher 
level than the lateral ones (fig. 135); soon afterward the lateral and median cells , 
"are at the same level (fig. 136); still later the lateral ones lie at a higher level 
г 
ص 
than the median ones (fig. 140). This is, of course, a result of the overgrowth, 
whereby the cells which were lateral in the blastopore lip come to overly those 
which were median in position. 
At the same time that these muscle cells are dividing the pair of large mesen- 
chyme cells, B*?, divides, the spindles being obliquely antero-posterior and dorso- 
ventral in direction (figs. 135, 136). This division is approximately equal and 
non-differential, and gives rise to the mesenchyme cells B”, B*'*, which lie on each 
side of the caudal endoderm cells. 
While these divisions are proceeding in the mesoderm, and thereby advancing 
these cells to the ninth generation, the delayed seventh cleavage appears in the 
mesenchyme and endoderm cells. The first of these cells to divide is the most 
anterior mesenchyme cell (А?%); the spindles are here nearly dorso-ventral in 
direction, and the resulting daughter cells (A*", A*2) are of about the same size, 
though the dorsal cell contains more protoplasm than the ventral one, as Castle has 
shown. | | 
= Coincidently with these divisions two pairs of endoderm cells (A*? апа B?) 
divide, the spindles being approximately transverse in the anterior pair and antero- 
posterior in the posterior one (fig. 135). 
A little later the four endoderm cells which meet at the vegetal pole (A7, TH fig. | 
136) divide, the spindles being antero-posterior in direction. The last remaining ріг 
of endode ar cells of the seventh generation to divide is the lateral one (A*5, ip «e d ur 
137) 1 have not seen this cell in division, Ре it is — ШТ. а exi 
9 JOURN. А. М. S. PHILA., VOL. XIII. 
