214 



INTRODUCTION TO CYTOLOGY 



egg. The two mitoses in the oocyte are meiotic in character and result in 

 a reduction from the zygotic to the gametic number of chromosomes. 

 They are frequently called "maturation divisions." 



From the foregoing it is evident that the animal ovum develops most 

 of the features characterizing it as a gamete before the meiotic divisions 

 take place. It will be seen later (p. 239) that in some cases the sper- 

 matozoon enters the egg before these divisions are completed. It is a 

 further significant fact that in many animals the differentiation of visibly 

 distinct cytoplasmic regions having a relation to parts of the future 



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Fig. 129. — First division of spermatocyte of Euschistus, showing behavior of chondrio- 

 somes (first row) and Golgi material (second row), ch, chromosomes. {After Bowen, 

 1920.) 



embryo may begin in the oocyte or ovum before the entrance of the 

 spermatozoon; in other words, embryogeny may actually begin before 

 fertilization (p. 419). 



The ovum is surrounded by a delicate vitelline membrane, which 

 becomes conspicuous only after fertilization, a thicker layer often showing 

 radial structure (vertebrates) and sometimes a third layer of structureless 

 jelly. The surrounding cells may add certain secondary layers, and later 

 there may be deposited one or more protective envelopes, such as the 

 albumen, shell membrane, and shell of the bird's egg. 



Spermatogenesis in Animals. — In the male animal those cells (sperma- 

 togonia) in the testis whose ultimate descendants are to become sperma- 

 tozoa multiply by divisions of the ordinary type until a certain number are 

 produced. These cells, now called primary spermatocytes, enlarge and 



