CLEAVAGE 221 



tive or typical form, the figure thus established is a hollow 

 sphere, the wall of which is composed of a single layer of cells 

 or blastomeres. This structure, however its actual form may 

 deviate from this type, is termed the blastula, and the cavity 

 within is the blastoccel, or segmentation cavity (Fig. 105). The 

 diverse forms of the blastula depend immediately upon the 

 arrangement of the preceding cell divisions; the blastula may in 

 some cases be almost or quite solid, so that the blastocoel exists 

 only virtually. 



About the time the blastula is formed the successive cleav- 

 ages have reduced the cell size to a physiological minimum and 

 thereafter the daughter cells increase in size subsequently to 

 each division, and there is no further reduction in the size of 

 the blastomeres; the volume of the cytoplasm, as well as of the 

 nucleoplasm, commences to increase, in other words the organ- 

 ism begins to grow. The relative time of the appearance of this 

 growth phase is widely different in different forms; in the Echi- 

 noids it appears when about sixty-four cells have been formed 

 (Godlewski). While there is no general and externally visible 

 indication marking a definite close of the cleavage period, the 

 formation of some type of blastula, or the initiation of cyto- 

 plasmic growth, is more or less arbitrarily assumed to mark its 

 termination, although many of the processes characteristic of 

 this phase of development, including of course cell division, 

 may continue for some time longer. We may now define cleav- 

 age as that early period of development characterized externally 

 by a rapid and orderly succession of mitoses, which results in 

 the formation, from the zygote, of a regularly arranged group 

 of small blastomeres possessing relatively large nuclei. 



In most cases there is a marked tendency for the blastomeres 

 to assume a spheroidal form, more or less modified by the 

 tension with which the cells are held together, by the pressure 

 of the egg membranes, etc. Sometimes the cells round up so 

 as to become practically spheres, in contact with one another 

 by greatly restricted surfaces (Amphioxus, Echinoderms, Ccelen- 

 terates). In other instances (frog, chick, Arthropods) the 

 separations between the blastomeres are mere grooves or fur- 



