JENNINGS: DEVELOPMENT OF ASPLANCHNA HEKEICKII. 81 



The inward movement of the large cell of quadrant Z>, 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 sui'face 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 karyokiuesis. The large cell, 

 in virtue of its mere size, conducts itself on the whole passively with 

 relation to tlie 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 wliich 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 hj Zelinka and Tessiu, 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 followiug 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. Tf the blasto- 

 meres should separate and project abf)ve 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 karyokiuesis, 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 question 

 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 



VOL. XXX. — NO. 1, 6 



