CHAPTER V. 



CLEAVAGE— (SEGMENTATION) . 



Following fertilization and the commingling of male and female chromo- 

 somes, there occurs the usual longitudinal splitting of these chromosomes as in 

 ordinary mitosis. One-half of each chromosome now passes toward each 

 centrosome. The result is that one-half of each male chromosome and one- 

 half of each female chromosome enter into the formation of each of the two 

 new, daughter nuclei (Fig. 22, 4, 5 and 6). The phenomena which follow are 

 apparently identical with those of ordinary mitosis and result in two similar 

 daughter cells. Each of the latter next undergoes mitotic division. In this 

 manner are formed four cells, eight cells, sixteen cells, and so on. This early 

 multiplication of cells which follows fertilization is known as cleavage or 

 segmentation of the ovum, the cells themselves are known as blastomeres and the 

 cell mass as the morula. 



While the object of cleavage and its results — the proliferation of cells from 

 the fertilized ovum and subsequent growth and development of the embryo — 

 also the general features of the process, are essentially similar in all eggs, 

 marked variations in details of cleavage occur in eggs of different forms, 

 apparently dependent largely upon the amount of yolk present and its distribu- 

 tion within the egg. (See page 12.) 



We distinguish the following : 



FORMS OF CLEAVAGE. 



a. Equal — e.g., alecithal eggs of Sponges, 

 Echinoderms, some Annelids, 

 some Crustaceans, some Mol- 

 lusks, Amphioxus, Mammals. 



b. Unequal — e.g., telolecithal eggs of 

 Cyclostomes, Ganoid 

 Fishes, Amphibians; usual 

 type in Annelids and Mol- 

 lusks. 



a. Superficial — e.g., centrolecithal eggs of 

 Arthropods. 



b. Discoidal — e.g., telolecithal eggs of 

 Cephalopods, Bony Fishes, 

 Reptiles, Birds. 



42 



Holoblastic (complete or total) 



Meroblastic (incomplete or partial) 



