REPRODUCTION 



175 



mechanism, other sperms are excluded. The sperm nucleus and egg 

 nucleus arrange themselves side by side; their chromatin undergoes 

 changes in arrangement, assuming the form of a spireme and then of 

 chromosomes; a spindle is formed and the zygote divides by mitosis. 

 This sequence of events indicates that the egg is induced to divide be- 

 cause of some stimulus given to it by the sperm. The nature of fertiliza- 

 tion is discussed in the later pages of this chapter. 



Sexual reproduction is concerned with the initiation of 

 the long process of development which results in the forma- 

 tion of the individual. It is not concerned with the mul- 

 tiplication of the cells of the animal body. The processes 

 which follow fertilization in metazoa, as well as the late 

 history of the germ cells, are treated in Chapter X under 

 the general title Embryology. 



Sexual Reproduction in Protozoa. — In the Rhizopoda 

 (Amoeba and others having pseudopodia) , so far as sexual 

 reproduction is known, union between similar individuals 



A B 



Fig. 133. — Sperm cell and ovum. A, spermatozoon of badger; D, fertilized ovum of 

 Nereis with two polar bodies, ph. {A from Dahlgren and Kepncr's Principles of Animal 

 Histology; B from Wilson's The Cell. Courtesy of Macmillan Co.) 



is the rule. In many of the species where it has been observed it 

 apparently occurs only at rare intervals. 



The Mastigophora (flagellate Protozoa) produce gametes which may 

 be of the same size or of different sizes. If the gametes are of the same 

 size they are said to be isogametes. In this case, however, the gametes 

 are perhaps alike only in size, since it is conceivable that there are internal 

 differences which are not visible. The gametes may both be motile or 

 the smaller only may be motile. In Gonium pedorale the gametes are 

 of the same size. They are produced by the division of any cell of the 



