JENNINGS: DEVELOPMENT OF ASPLANCHNA HERRICKII. 81 
The inward movement of the large cell of quadrant D, rather than that 
of the other cells, seems due to several causes: — (1) The inequality of 
the cells. The large ventral cell having a much greater radius of 
curvature has less surface tension, and therefore may more easily change 
its form. () It thus yields to pressure, and fits itself to the changing 
form of the smaller cells. These are thus able to creep over it, as it 
were, and surround it. The greater quantity of cytoplasm in the 
entoderm cell as compared with the size of the spindle seems also to 
result in less change of form at the time of karyokinesis. The large cell, 
in virtue of its mere size, conducts itself on the whole passively with 
relation to the more active smaller cells. (2) The sequence of cleavage 
is possibly connected with this. At a given cleavage the large cell 
divides first, so that, when the karyokinetic stretching of the other cells 
takes place, the entodermic cell is in a resting condition, and therefore 
passive. (3) The direction of the spindles, which is prevailingly dorso- 
ventral, results in a continued dorso-ventral extension of the cells, so 
that invagination would naturally take place at one of the two ends. 
The developing egg may be likened to a fountain in which there is an 
upward movement within, an outflow above, at the animal pole, and 
a downflow about the periphery. 
The enclosure of the large ventral cell of quadrant D is what has been 
considered gastrulation proper by Zelinka and Tessin, only the products 
of this cell being spoken of as entoderm. But after this enclosure is 
complete the process continues, unchanged in character, the ventral cells 
of the other quadrants following that of quadrant D to the inside of the 
embryo, as shown in Figures 76-79 (Plate 9). 
A necessary condition for all this displacement is of course the 
retention by the egg as a whole of its form and outline. If the blasto- 
meres should separate and project above the general level, in the manner 
that is common for the cells of mollusks (see the figures of Unio by Lillie, 
'95) at the time of karyokinesis, no compensating inward movement of 
the other cells would be necessary, and apparently therefore gastrulation 
would not take place. The retention of the regular form appears thus 
to be of the highest, importance for the development, and the questioh 
arises as to how this form is preserved. As previously stated, no mem- 
brane is visible; and any uniform force, such as surface tension or a 
centripetal attraction, would produce a spherical instead of an ellipsoidal 
form. The development of the egg proceeds as if it were enclosed in a 
rigid mould of oval or ellipsoidal form, so that the contents of the mould 
are rotated, while the form is retained. The retention of this shape 
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