i82 ZOOLOGY FOR MEDICAL STUDENTS chap. 



oviduct this rachis breaks down so that the cells now lie loose in the 

 cavity. 



In the male the arrangement is similar as regards the most important 

 points — the arrangement of the cells round a rachis and the direct con- 

 tinuity of the thread-like gonad with the tubular duct. But there is 

 only a single gonad or testis in the male. Its duct serves as a seminal 

 vesicle, in which the microgametes accumulate, and this opens not 

 directly to the exterior but into the floor of the alimentary canal near 

 the anus. The terminal piece of the alimentary canal into which the 

 male duct opens has on its dorsal side two curved forwardly-projecting 

 pockets each of which secretes a strong chaeta. These chaetae can be 

 protruded through the genito-anal opening and are inserted into the 

 external genital opening of the female at the time the microgametes are 

 transferred. 



Gametogenesis and Fertilization 



The reproductive organs of Ascaris are of special interest and im- 

 portance from the fact that their study provided the foundation for 

 much of our present-day knowledge regarding the origin and develop- 

 ment of the gametes and their union in the process of syngamy or fer- 

 tilization. We shall now make use of them in giving a description of 

 these processes. 



It has already been indicated that one of the chief characteristics 

 of mitotic nuclear division is the concentrating of the nuclear material 

 or chromatin into special little masses named chromosomes. What 

 has not been mentioned, so far, is that the number of these chromosomes 

 in the dividing nucleus is as a rule fixed and definite in the cells of any 

 particular species of animal. This holds even for the period when the 

 individual consists of a single cell or zygote. But seeing that the zygote 

 itself arises in syngamy by the fusion of two gametes it necessarily follows 

 that the gametes must contain each only half the normal number of 

 chromosomes. 



It thus comes about that there are two chromosome numbers char- 

 acteristic of the species (i) the number characteristic of the ordinary 

 cells of the body — known as the diploid number and (2) the number — 

 half as great as the former — characteristic of the gametes and known as 

 the liaploid number. 



The characteristic (diploid) number of chromosomes in a particular 

 species of animal may be very large, as many as 168 in a little fresh-water 

 crustacean (Artemia), while on the other hand it may be comparatively 

 small. In Ascaris megalocephala the number is only four (in the case 



