182 ZOOLOGY FOR MEDICAL STUDENTS CHAP, 



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



In tin- 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 

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

 in which the microgametes accumulate, and this opens not 

 v 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 

 rred, 



GAMETOGENESIS AND FERTILIZATION 



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- 

 tili/ation. \Ye shall now make use of them in giving a description of 



processes. 



It has already been indicated that one of the chief characteristics 



<>f mitotic nuclear division is the concentrating of the nuclear material 



or ( hromatin into special little masses named chromosomes. What 



>t 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 



in s\ iiLamy by the fusion of two gametes it necessarily follows 



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



chromosoi 



It thu-. (nines about that there are two chromosome numbers char- 

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

 i the body known as the diploid number and (2) the number 

 as the former characteristic of the gametes and known as 

 iploid number. 



"t eristic (diploid) number of chromosomes in a particular 



imal may be very large, as many as 168 in a little fresh-water 



;/>/), while on the other hand it may be comparatively 



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



