V ASCARIS 183 



of a particular variety of this species only two) and the smallness of this 

 number is one of the main factors which have facilitated the follow- 

 ing out of the details of the processes involved in gametogenesis and 

 fertilization. 



The study of sections through the long thread-like gonad of A. 

 megalocephala shows its cells to be in a state of active multiplication, 

 and each mitotic nucleus presents the diploid number of chromosomes — 

 four (Fig. 83, A). This holds down to a certain level in the gonad but 

 then there comes a remarkable change, as from now onwards each mitotic 

 nucleus shows only two, i.e. the haploid number of chromosomes. Further, 

 each individual chromosome has undergone a complication of its struc- 

 ture and has now the appearance of a bundle of four beaded rods in 

 close apposition (Fig. 83, B). Each of these complex chromosomes is 

 known as a tetrad from its quadripartite 

 nature. 



The reduction of the number of 

 chromosomes from diploid to haploid — 

 meiosis as it is termed — a necessary 

 forerunner to the process of syngamy, is 

 clearly a phenomenon of great import- 

 ance and the interesting question arises — 

 How is the reduction brought about ? '°' '' 



, . Nuclei from the testis of Ascaris. 



A possible explanation readily SUg- a, before and B, after the formation of 



gests itself. It will have been noticed ^^^. ^^'^^^- <?"= right-hand tetrad in 



° ^ B IS seen m end view.) 



in Fig. 83, A, that the four chromo- 

 somes present before meiosis are split longitudinally. Now if these 

 chromosomes were to become shortened and straightened and then 

 come together side by side in pairs they would clearly give rise to the 

 haploid number of tetrads. That this is a correct explanation is 

 testified by a rare abnormality which has been observed in which two 

 of the rods composing the tetrad were distinctly different in length 

 from the other two. 



Its correctness is also testified by much evidence derived from crea- 

 tures other than Ascaris and the view is therefore now generally accepted 

 that the reduction in number of the chromosomes is brought about by 

 their coming together in pairs. This process of coming together is known 

 as syndesis. 



Male (Fig. 84, left-hand figures) 



The appearance of the tetrads is the inaugural phase of a mitotic 

 division. The external boundary of the nucleus disappears and a spindle 



