64 ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 
This stage is important as marking the beginnings of gastrulation for which 
preparations were made in preceding stages. Тһе endoderm cells and the four 
posterior mesenchyme cells whieh remain in the seventh generation now lie at a 
considerably lower level than the surrounding cells. This, although usually spoken 
of as an invagination, can scarcely with right be called such, for as sections show 
there is neither at this stage, nor at any preceding one, any considerable blastocoel ; 
since there is no cavity into which the cells can. push 1t is scarcely permissible to 
speak of their invaginating. Іп reality the gastrulation is due to two factors, 
neither of which is invagination. Тһе first апа most important is the change of 
shape of the cells, which has been described in part already; the second is the 
overgrowth of the cells lying around the endodermal area. 
As to the first we have already seen that in the 16-cell stage the cells at both 
poles are of about equal height; during the fifth cleavage the cells at the animal 
pole become long and columnar, while those at the vegetal pole are broad and flat ; 
during the sixth cleavage the cells at the two poles change shape so that at the 
close of this cleavage (64-cell stage) the cells of both poles are of nearly equal 
height, those at the vegetal pole being perhaps slightly longer than those at the 
animal pole. At this stage the cells of the dorsal (vegetal) hemisphere still have a 
larger surface area than those of the ventral (animal) hemisphere, so that when 
viewed from the vegetal pole only cells of the dorsal hemisphere can be seen, but 
when viewed from the opposite pole a peripheral row of dorsal cells can be seen 
around those of the ventral hemisphere (figs. 150, 151). During the seventh 
cleavage this change of shape progresses rapidly so that at the 76-cell stage the 
surface area of the dorsal cells is less than that of the ventral ones; the endo- 
derm cells in particular grow long and narrow, whereas the ectoderm cells 
become broad and flat (figs. 198-204). After the seventh cleavage of the ecto- 
derm and mesoderm cells (112-cell stage) a row of ventral hemisphere cells is 
visible all around the periphery of the dorsal hemisphere when the embryo is 
viewed from the dorsal pole (figs. 133, 154). The remarkable reduction in the 
surface area of the endoderm cells, which occurs without any division in these cells, 
and wholly by their change of shape will be best appreciated by comparing figures 
131 with 134, and figures 197 with 200; in all of these figures the endoderm cells 
are in the seventh generation, but the superficial area of these cells in the two older 
stages is not more than half as great as in the two earlier ones. In proportion as 
these cells decrease in surface area they increase in depth, their inner ends become 
enlarged, and at the same time their nuclei are withdrawn from the surface to a 
deeper level in the cells (text figs. XXI-XXIV). The flattening of the ectoderm 
cells leads to their covering a larger and larger surface area until they finally over- 
grow the marginal cells of the dorsal hemisphere. Samassa undertook to explain the 
columnar form of the cells at the animal pole in the 32-cell stage by the pressure 
exerted on them by the overgrowing cells of the opposite pole, but it is obvious 
that neither at this early stage, nor during the later опе just deseribed, can the 
cause of this change of shape be located in the cells of one hemisphere rather: than 
in those of the other. 
