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ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 31 
than the others, must continue to divide more rapidly and thus give rise to the more 
numerous ectoderm cells of the gastrular stage. So far from there being any dem- 
onstration of this proposition there ік actually no evidence offered in support of it. 
Furthermore, I can affirm from my own studies that it is not frue. The cells which 
lag behind in division up to the 64-cell stage, thereafter divide much more rapidly 
than the others and. give rise to the ectoderm of the gastrula (cf. figs. 130-154 and 
196-204). 
(2) Castle’s second reason for rejecting the orientation of Van Beneden and Julin 
is the same as Samassa’s, vzz., the peculiar shape of the cells at the two poles. In the 
32-cell stage and even earlier the cells at the maturation pole are long and columnar 
while those at the opposite pole are thin and superficially large. “They [the colum- 
nar cells] retain this columnar form up to and throughout gastrulation” (1896, p. 
237). They thus give rise directly to the columnar endoderm cells which are ulti- 
mately invaginated. On the other hand, Van Beneden and Julin maintained that the 
flattened cells of the 32-cell stage became the columnar cells of the 44-cell stage and 
that the columnar cells of the earlier stage became the flattened ones of the latter 
stage. Castle says that their figures show at a glance the absurdity of such an 
interpretation (1894, p. 208; 1896, p. 237). Since the whole orientation which һе 
adopts as opposed to that of Van Beneden and Julin rests upon the establishment 
of this one point, it passes belief that he, as well as Samassa, should not have taken 
the most evident and direct step to prove it. Van Beneden and Julin figure optical 
sections in the sagittal plane of an egg in the 32-cell stage showing the eolumnar 
cells at the ventral pole, and of one in the 44-cell stage showing them at the dorsal 
pole. Castle figures actual sections of a 32-cell stage and of a 76-cell stage, but 
none between these two. A study of actual or of optical sections of eggs transi- 
tional between the 32-cell and the 76-cell stages would have shown conclusively 
that the columnar cells of the former are gradually transformed into the flattened 
cells of the latter, and the flattened cells of the one into the columnar cells of the 
other, and would thus have completely established Van Beneden and Julin’s 
orientation. Such a series of optical sections of the Czona egg, viewed from the left 
side and also from the posterior pole, is shown in text figures ІХ to XVI, and the 
various stages in this change of shape can there be clearly followed. А similar 
series of actual sections of the egg of Cynthra is shown in text figures XVII to XXIV. 
I do not find that this transformation is quite as rapid іп Cynthza and Czoza as is 
indicated by Van Beneden and Julin's figures Әс and 10 c fof CZave//zma. At the 
44-cell stage the cells at both poles are columnar and of nearly equal height (text 
figs. ХПІ, XIV), and not until the 64-cell or even the 76-cell stage is this trans- 
formation complete. It must not be supposed, however, that this change in shape 
of the cells at the two poles is a continually progressive one, since all the cells be- 
come more superficial during division and more columnar during rest. Consequently 
every cell changes shape more or less during each cycle of division; this is well 
shown in figures ХУШ and XX. — 
Other details which Castle regards as confirmatory of his view will be taken 
