STUDY OF CELL MECHANICS 463 



equator of the egg and just beneath it the cleavage furrow will 

 appear. This is a very constant feature in Strongylocentrotus 

 and has been noted by Boveri, Goldschmidt and Popoff ('08), 

 and many others. 



In monaster eggs we have the same swelling or blistering of 

 the ectoplasmic layer, but it appears, not in a girdle between 

 the two asters, but on the whole side of the egg opposite the 

 chromosome side of the aster. 



It is the third feature, perhaps, which forms the most striking 

 parallel between the two types of eggs, namely, the behavior 

 of the granular cytoplasm. In normal eggs, just beneath the 

 swollen hyaline layer, the cleavage furrow appears. This 

 gradually separates the egg into two cells, but the walls of the 

 cleavage furrow are not as smooth as they are ordinarily fig- 

 ured. If any egg is observed very closely under moderate 

 powers of the microscope, the furrow walls will be found to be 

 undergoing changes during which little ridges and depressions 

 appear. Finally, as the asters disappear the two new blasto- 

 meres* round up and all active external movement ceases. 



In monaster eggs, following immediately the swelling of the 

 ectoplasmic layer the peculiar movement of the protoplasm 

 sets in, and this leads to changes in surface contour and to the 

 cutting off of protoplasmic masses, both of which indicate that 

 this side of the egg is undergoing intense changes in surface ten- 

 sion. As the astral radiations disappear, these movements 

 become less intense and eventually the normal rounded form 

 of the egg is resumed. 



It 'is clear- that the force, or forces, which operate to bring 

 about the separation of the two blastomeres with the attending 

 movement of the furrow walls in normal cleavage are repre- 

 sented in the changes in surface contour and the cutting off 

 of protoplasmic balls in monaster eggs. 



In the second and later division cycles one finds in the mon- 

 aster eggs, at least as far as the ectoplasmic layer and cyto- 

 plasm is concerned, the same correspondence to normal cleavage 

 as described above. Thus, in those monaster eggs, which at the 



