426 CASWELL GRAVE 



the nucleus into two groups. That group which occupies the 

 portion of the cell between the nucleus and the base of the cell, 

 is composed of a relatively small number of spherules each 

 surrounded by, or suspended in, a relatively larger amount of 

 cytoplasm than is to be found between the spherules compos- 

 ing the larger group included within the portion of the cell 

 situated internal to the nucleus. 



The cells in mitosis are approximately spherical in form. 

 The yolk spherules during this phase are arranged around the 

 dividing nucleus in a continuous peripheral zone, which, on 

 the side of the cell nearest the basement membrane, is one spherule 

 in thickness and increases in thickness to three or four spherules 

 on the opposite side. 



The position of cells in mitosis within the epithelium of the 

 blastula in all cases observed, is at the periphery. That part 

 of the cell wall which is in contact with the basement mem- 

 brane of the blastula, apparently forming a part of this limit- 

 ing membrane, retains its original flat contour during the divi- 

 sion of the cell, showing that this part of the cell is firmly attached 

 or cemented to the basement membrane, or to the bases of the 

 adjacent cells, and that the cells of the blastula are not free to 

 move about or change their positions with reference ,to one 

 another. 



The relatively enormous change that takes place in the shape 

 and position of a cell during mitosis greatly affects the form 

 and condition also of the cells adjacent to it. The latter, in 

 their adjustment to their spherical dividing neighbor cell, be- 

 come bent, twisted and constricted and their nuclei become flat- 

 tened or pushed from their usual positions, as shown by figure 2. 



Yolk segregation. It would seem that the larva is in some 

 way disturbed by the outgo of energy, and the hindrance to 

 development, involved in the juggling of its yolk material, 

 for, when we come to a later stage in the development of Ophiura, 

 (the late gastrula in which the alimentary canal and coelom 

 become differentiated) we find by an examination of the cells 

 that make up its epithelia and form a part of the contents of 

 the segmentation cavity, that a complete change in the dis- 



