CHAPTER VI 

 CLEAVAGE 



THE grosser and externally visible processes of development 

 begin with the cleavage of the fertilized ovum, or zygote. The 

 period of cleavage therefore may be regarded as the second of 

 the " grand periods" in individual history. During the first 

 general period occur all the events leading up to and including 

 the final establishment of the zygote, a single cell, but a new 

 organism. During this second period the Metazoan really 

 becomes made up of many cells. 



The essential process underlying many of the varied phenom- 

 ena of cleavage is a process already familiar, mitotic cell 

 division; but it is true that cell division continues long after the 

 cleavage period proper is terminated, in some tissues through- 

 out the life of the organism. And as we shall soon see, the 

 process of cleavage involves a great deal more than merely a 

 succession of cell divisions. 



Certain general characteristics of the mitoses of the period 

 of cleavage, or segmentation, of the zygote, may be observed, 

 but it is difficult to state precisely wherein these cell divisions 

 differ from those of later development. Probably the most sig- 

 nificant characteristic of the divisions of this period is that they 

 are rarely at random, but nearly always occur in an orderly 

 fashion, according to a definite schema or plan, which is quite 

 fixed for each species or larger group, and which involves the 

 entire cell community. The mitoses of cleavage are fre- 

 quently very unequal and the daughter cells may be very unlike, 

 not only in size, but further as regards cytoplasmic character 

 and the nature of various cell inclusions, which may be distrib- 

 uted dissimilarly during these divisions. After the first few 

 mitoses the blastomeres may not divide synchronously, so that 

 the regular and rhythmic geometric increase in their number 



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