222 GENERAL EMBRYOLOGY 



rows on the surface of the mass; here the cells are broadly in 

 contact and in some instances remain connected by very 

 delicate protoplasmic bridges similar to those connecting tissue 

 cells previously mentioned. In a few instances (some Arthro- 

 pods, a few Coelenterates) these early cell divisions may be 

 imperfect, the nuclei alone dividing and forming a syncytium; 

 later the cytoplasm also divides simultaneously into a number 

 of complete cells. Or cells once formed may fuse into syncytial 

 masses, as in some Crustacea. 



The internal processes of development occurring during this 

 period are of greater importance than the external phenomena. 

 As stated above (Chapter V) one of the underlying processes 

 of great importance seems to be the synthesis of chromatin 

 which occurs at this time. 



In the opinion of many this is a highly characteristic chemical process 

 of early development. It results from rapid oxidations within the egg 

 which were made possible by the transformation of the egg membrane 

 from a condition of relative impermeability, to a state of high permea- 

 bility, oxygen thus being readily admitted from without. This trans- 

 formation is thought to result from the chemical reactions of the 

 cytoplasm following fertilization, during which there occurs also the 

 activation, or perhaps the introduction, of specific enzymes which bring 

 about this characteristic oxidative synthesis. 



This view as to the chemical process most essential in cleavage 

 agrees well with the "kern-plasma" theory of Richard Hertwig, accord- 

 ing to which, as already mentioned, the ovum and zygote are to be 

 regarded as instancing abnormal or especially adapted relations between 

 nucleus and cytoplasm ; for here the relative amount of cytoplasm is far 

 in excess of the common proportion. In cleavage, with its proportional 

 increase in nuclear substance, we should see a restoration to or toward 



Volume 



the normal of the kern-plasma ratio nucleus ( V A 



Volume \F C / 



cytoplasm 



Another internal process is of prime importance. We 

 have already become familiar with certain facts that one 

 result of maturation and fertilization is the presence in the 

 zygote of two similar chromosome groups, derived respectively 

 from the male and female parents; that while the egg nucleus 



