HISTOLOGY AND CYTOLOGY 373 



out near the periphery. A splitting of the chromosomes occurs, 

 and the anaphase is entered upon. With the consequent division 

 of the cytoplasm comes a marked difference from sperm formation. 

 Instead of the cell dividing equally it splits into one large cell, and 

 one very small one that contains little more than a haploid group of 

 bivalent chromosomes. These two daughter cells may be termed 

 the secondary ooeytes, but, on account of their striking difference 

 both in size and fate, the large cell is spoken of as the ovum, and 

 the small one as the polar body. In many animals no further 

 changes are undergone while the egg remains in the ovary, and it is 

 not until after it is shed that the next division occurs. This division 

 is an ordinary and not a reducing one, and, as far as the distribution 

 of the cytoplasm is concerned, is similar to the one preceding it. 

 That is to say, the result is a large cell, now termed the mature ovum, 

 and another, or second, polar body. The first polar body may also 

 divide into two, and although it does not always do so, it may be 

 considered that it should divide in typical and primitive cases. 

 Thus from the primary oocyte there are produced four cells which, 

 however, are not equal in size and potentialities, as are the sperma- 

 tids, but consist of one large mature ovum capable of development if 

 fertilised, and three minute polar bodies which cannot develop and 

 have, as it were, sacrificed their cytoplasm to produce one large ovum. 

 The relation between the two processes of germ cell forma- 

 tion, or, as they are collectively termed, the maturation o! the gametes 

 in the male and female, can be briefly set out in the following way : 

 They are alike in 



(a) Their nuclei contain the haploid number of chromosomes 

 the result of a meiotic and one subsequent division. 



(b) Their chromosomes are alike in form, size and with few 

 exceptions in number. 



(c) Generally they can develop only after union. 

 They differ in 



Spermatozoon. Ovum. 



(a) Little cytoplasm. Much cytoplasm. 



(b) No food or yolk. Always contains yolk and 



sometimes very large amount. 



(c) Actively motile. Non-motile. 



(d) Centrosome present. Centrosome absent. 



(e) One of four similar cells One of four dissimilar cells 

 derived from the spermatocyte derived from the oocyte, the 

 all being functional. other three non-functional. 



(f) Usually completely ma- Usually formed in ovary, but 

 tured and formed in testis. not matured until after dis- 

 charged. 



