68 ORGANIZATION AND CELL-LINEAGE ОЕ ASCIDIAN EGG. 
cells and form the anterior border of the blastopore belong to the dorsal hemi- 
sphere; the rest of this plate is composed of cells of the ventral hemisphere, 
arranged in four rows of six each, and lying just in front of the chorda region ; 
therefore in the 218-cell stage the nerve plate consists of forty cells arranged in six 
transverse rows, each containing six cells except, the two most posterior rows which 
contain eight each. The animal pole, with the polar bodies still attached, is still 
situated back of the middle of the ventral face, six cell rows posterior to the 
anterior edge of the neutral plate; there are but five rows of cells of the ventral 
hemisphere posterior to the animal pole, while there are ten such rows in front 
of it. 
V. LATER DEVELOPMENT. 
My observations on the later stages of development agree in the main with 
those of previous workers and particularly with those of Castle, who made a thorough 
study of these stages by means of serial sections. It is true of these stages, how- 
ever, as it is of the cleavage stages, that many topographical relations can be made 
out more satisfactorily by a study of entire preparations. I have therefore 
devoted especial attention to such preparations, and my observation, both on living 
and on stained material, are embodied in plates V and X. 
1. Closure of Blastopore. 
During the closure of the blastopore the embryo changes shape and at the 
same time the egg axis is shifted. This stage is therefore an important one іп 
the orientation of the later stages. The gastrula is at first disk-shaped (fig. 134), it 
then becomes saucer-shaped (fig. 136) and then cup-shaped (figs. 144, 145). During 
this change as the embryo increases in depth it decreases in its other dimen- 
sions so that it becomes more nearly spherical (fig. 145). The closure of the 
blastopore takes place more rapidly from the anterior than from the posterior 
side; in fact after the general drawing together of the margins of the saucer- 
shaped gastrula the posterior lip remains nearly stationary until the last stage in 
the closure of the blastopore. 
Soon after the 218-cell stage the gastrula becomes elongated and egg-shaped, 
the posterior end being somewhat narrower than the anterior. Тһе anterior lip of 
the blastopore continues to grow posteriorly while the lateral lips draw nearer 
together; thus the blastopore becomes T-shaped (fig. 148), and finally, by the 
further growth of the anterior lip, the anterior part of the blastopore, represented 
by the bar of the T, is covered and the blastopore is reduced to a longitudinal 
groove between the laterallips (figs. 152, 153). In the growth of the lateral lips 
they come to lie at a higher level than the anterior lip, and consequently as the 
latter continues to grow posteriorly, the former are tilted up at their anterior 
ends until they become vertical in position. These lateral lips are at first formed 
only of the muscle cells, but later the ectoderm cells completely overgrow them. 
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