MATURATION. 27 



polar body (Figs. 9, 12, 13 and 17). In some cases the first polar body divides, 

 giving to each resulting cell the reduced number of chromosomes. There have 

 thus resulted from the reduction division in the oocyte, three or four cells 

 (Fig. 17). One of these is the mature ovum containing one-half the somatic 

 number of chromosomes, the other two or three are the cast off and apparently 

 useless polar bodies, each of which contains one-half the somatic number of 

 chromosomes. The primary spermatocyte, on the other hand, gives rise to 

 four cells which are equal in size as well as in their chromatin content (Fig. 17). 



There thus develop from both spermatocyte and oocyte four structures, 

 each containing one-half the somatic number of chromosomes, only, as already 

 noted, in the case of the spermatocyte, all four of the resulting products of 

 division become functional as spermatozoa, while in the case of the oocyte only 

 one product, the mature egg cell, becomes of functional value. (Compare 

 Fig. 17 a and b.) 



The time of formation of the polar bodies differs for different eggs. In 

 some cases both polar bodies are extruded before the entrance of the spermato- 

 zoon. In other cases one polar body is formed before, the other after the 

 entrance of the spermatozoon. In still other eggs both polar bodies are formed 

 after the entrance of the spermatozoon. 



From the above description it is evident that the phenomena of maturation 

 are essentially similar, and the process itself is identical in both male and female 

 sex cells. The details of the process vary, but the result — the reduction of the 

 number of chromosomes in the mature sex cell to one-half the number char- 

 acteristic for other cells of' the species — is always the same. 



The exact manner in which reduction takes place has been the subject of 

 much investigation and controversy and has as yet been determined for com- 

 paratively few forms. In the higher animals the point at which the actual 

 reduction in number of chromosomes takes place is usually two generations 

 of cells before the formation of the mature sex cells. The behavior of the 

 chromatic spireme of the oocyte or of the spermatocyte, as it breaks up into one- 

 half the somatic number of chromosomes, varies sufficiently to allow two 

 general types of maturation to be distinguished, known, respectively, as reduc- 

 tion with tetrad formation and reduction without tetrad formation. 



In reduction with tetrad formation the spireme (primary oocyte or primary 

 spermatocyte) segments into the reduced number of chromatin masses, each one 

 of which divides into four pieces and is consequently known as a tetrad. The 

 two maturation divisions now follow, the second following the first rapidly with- 

 out reconstruction of a nuclear reticulum. In the first of these each tetrad 

 divides equally, giving rise to two dyads, each consisting of two pieces of 

 chromatin and each passing to one of the daughter cells (secondary oocyte, 

 first polar body, spermatocyte) (Figs. 9 and 14). In the second maturation 



