44 bulletin: museum of comparative zoology. 



would naturally, if the cell be a body not entirely fluid, give it a rotary 

 motion. As a result of the partial rotation, the cloud of granules is 

 now found underneath the anterior cells (Plate 6, Fig. 50). When the 

 anterior cells extend ventrad, the form of d?-^ is so changed by its 

 change of position that the cells do not push against it, but glide over 

 it, at the same time vacating a part of the dorsal region of the egg 

 (Plate 8, Fig. G-t). Therefore tlie cell cf-^, as it moves inward, con- 

 tinues its rotation in the same direction as before, and the cloud of 

 granules is brought to the dorsal end of the cell, and even farther. 



Such a rotation would explain clearly the peculiar position of the 

 spindle in cf^^ at the sixth cleavage. To agree with the other cells of 

 the ventral layer, the spindle would have to take a dorso-ventral posi- 

 tion. (Compare Fig. 55, Plate 7.) But a rotation from posterior to 

 anterior just before this cleavage would bi'iug the spindle into an antero- 

 posterior position, such as actually occurs. 



However, the explanation is not very satisfactory, for several reasons. 

 (1) We have previously seen that the granules forming the granular 

 cloud do move within the cytoplasm of the cell, so that this change of 

 position of the granular mass may be due to simple migration through 

 the protoplasm of the cell. (2) A study of the movements of the 

 asters after the fifth and sixth cleavages does not give evidence of any 

 such rotation. (See the account of the movements of the asters before 

 the sixth cleavage, page 36, and before the seventh cleavage, page 54.) 

 (3) Such a rotation explains the position of the spindle at the sixth 

 cleavage only, — while all the later cleavages of the inner cells are like- 

 wise out of relation to the divisions of the outer cells. (4) The ex- 

 planation assumes that the position of the axes of cleavage is definite, 

 and determined within the cell itself, so that, if the cell rotates, the axis 

 rotates with it, — which is not proved. 



The only conclusion in which we are entirely justified is therefore 

 merely this, that as the relation of the cell d^-^ to the other cells and to 

 the egg as a whole becomes fundamentally changed, the method of di- 

 vision likewise becomes fundamentally changed. 



At the close of the processes thus far described, the egg has evidently 

 passed into the "gastrula stage" proper (Plate 8, Fig. 64). The blasto- 

 pore is still large and lies at the macromere end of the egg. It is sur- 

 rounded at first by eight cells, two belonging to each of the four original 

 quadrants of the egg (Plate 7, Fig. 56). Later, as the sixth cleav- 

 age is entirely fiuislied, one of the cells (c'"^) becomes displaced and is 

 shut out from the margin of the blastopore (Plate 8, Fig. 63), which is 



