CHAPTER III. 

 MATURATION. 



In the preceding chapter (p. 10) it was stated that in practically the entire 

 animal kingdom, and without exception in Vertebrates, the condition essential 

 to the production of a new individual was the union of two sexually different 

 cells. Before this can take place, however, the sex cells must pass through 

 certain preliminary and preparatory processes which are known collectively as 

 reduction of chromosomes or maturation. 



The manner in which this reduction takes place is in most sex cells extremely 

 difficult of demonstration, this being especially true in the higher forms, notably 

 in Mammals. The fact, however, that such reduction occurs in all cases in- 

 vestigated, where there is sexual reproduction, is undisputed. The result also 

 is invariably the same. The number of chromosomes in the mature sex cell is 

 reduced to half the somatic number for the species. 



MATURATION OF THE OVUM. 



Because of the difficulties of observing this process in higher forms, it is 

 advisable to describe first the maturation of the ovum in such a simple type as 

 Ascaris. This has become a classic for the study of maturation owing to the 

 fact that it shows the various stages of the process with remarkable clearness. 



In Ascaris at about the time the spermatozoon enters the ovum, the chromatic 

 elements of the nucleus are found collected into two groups and each group in 

 turn composed of four rod-shaped pieces (Fig. 9, A , B, C and D) . The groups are 

 known as tetrads, and the number of tetrads is always one-half the usual number 

 of chromosomes. In this particular form the usual number of chromosomes is 

 four. An achromatic spindle next forms as in ordinary mitosis, and two of 

 the chromatin rods from each tetrad pass out into a small' mass of cytoplasm 

 which becomes separated from the ovum (Fig. 9, E, F, G and H) as the first 

 polar body. The four chromatin rods which remain in the ovum constitute two 

 dyads; each dyad representing one-half of a tetrad. Closely following the 

 formation of the first polar body and without any return of the chromatin to a 

 resting stage, the second polar body is given off. This is accomplished by two 

 chromatin rods, one from each dyad, passing out from the ovum, surrounded by 

 a small mass of cytoplasm, as in the case of the first polar body. There thus 

 remain in the ovum two rods or two chromosomes, one-half the usual number 



17 



