158 GENERAL BIOLOGY 



half from the other, every cell in the resultant organ- 

 ism shares this double nature, that is, half its chroma- 

 tin is paternal, half maternal. 



Following the formation of the cleavage nucleus, 

 as the nucleus of the unicellular zygote is called, a 

 series of rapidly accomplished transformations en- 

 sues, which molds the undifferentiated cell into the 

 form characteristic of the species. These changes 

 may be grouped into three stages : 



First : A period of rapid cell multiplication un- 

 accompanied by growth. 



Second : A period of growth, together with - 



Third : A period of differentiation, during which 

 the tissues resulting from the first period become 

 changed from a generalized condition to one involv- 

 ing specification or specialization. 



These three periods cannot be sharply distin- 

 guished, one from another, particularly the second 

 and third. 



Cleavage. In the first stage, just mentioned, 

 the original zygote becomes subdivided again and 

 again into a great number of cells, which, with each 

 cell division, become smaller and smaller. The 

 energy for these repeated mitoses is found in the 

 reserve food material which is stored up in the egg- 

 cell, even before its maturation. In such animals 

 as have a very brief larval period, or in which the 

 larva is self-supporting at a very early age, the 

 amount of such reserve food, or " yolk," is much 

 less than is required by a form which has a long 



