OPHIURA BREVISPINA 425 



of the eggs of all of those echinoderms in which the supply of 

 yolk material is large. Some idea of the character and value 

 of material of this kind for such a study may be had from text 

 figure 2, A-F. 



YOLK MANIPULATION, A FACTOR IN DEVELOPMENT 



A factor or condition of development of considerable impor- 

 tance, present to a greater or less degree in the early develop- 

 mental stages of all animals, may be defined as the mechanical 

 operation of changing the position and arrangement of the 

 yolk material within each cell both before and after each cell 

 division. It involves the expenditure of a relatively consider- 

 able amount of energy in processes which can not be regarded 

 as essentially developmental in character. 



The existence of a condition such as that referred to is made 

 strikingly apparent in the egg of Ophiura during its early develop- 

 ment because of the presence and uniform distribution through- 

 out the cytoplasm of a relatively very large amount of yolk 

 material which must be manipulated by the cells of the segment- 

 ing egg. 



The relatively enormous amount of movement and rearrange- 

 ment of the yolk spherules which takes place in the cells, both 

 preceding and following every mitosis, may be inferred from 

 figures 2 and 3, which are from camera sketches of portions of 

 sections of a blastula fixed in an osmic acid solution; one cut 

 transversely to the long axis of the cells and passing through 

 the cells at the level of their nuclei; the other a section of cells 

 in the plane of their long axes. Each drawing shows a num- 

 ber of cells in their resting condition and two cells in mitosis, 

 one of the latter in a stage of division just at the close of ana- 

 phase. It may be noted that the cells not in mitosis have the 

 form of slender prisms in which the ratio of the length of the 

 long axis to that of the transverse is about as 9 is to 1. The 

 nuclei are situated near the basal ends of the cells, are oval in 

 outline and each almost fills the space between the side walls 

 of the cell. The yolk spherules of each cell are separated by 



