CHAPTER III 

 THE MORPHOLOGY OF FERTILIZATION 



Before beginning this subject it is necessary to pre- 

 pare the ground a Httle by certain considerations on 

 the germ cells themselves. To go at all fully into their 

 characteristics and history would require a volume in 

 itself, and the present considerations will be strictly 

 limited to what is necessary for understanding fertili- 

 zation. 



I. THE RELATION OF MATURATION OF THE GERM CELLS 



TO FERTILIZATION 



After the full growth of the ovum and during or 

 immediately before fertilization the ovum forms two 

 small cells, known as the polar bodies, by a process of 

 karyokinetic division (see Figs. 4 J, 8c, gd, pp. 56, 62, 

 and 66). If the first of those cells divides, as sometimes 

 happens, four cells are produced by the fully grown 

 ovum, three of which are rudimentary, viz., the polar 

 bodies, and the other is the mature ovum, which is not 

 appreciably reduced in size by the' divisions. The polar 

 bodies take no part in development, but degenerate, 

 though they frequently remain attached to the egg for 

 a considerable period of time. They are formed at that 

 end of the axis, or chief developmental gradient of 

 the ovum, which is known as the animal pole. 



The polar bodies are essentially abortive ova, and 

 the divisions by which they are formed correspond 

 precisely in their nuclear phenomena to the last two 



44 



