ORIGIN OF THE TETRADS 



251 



Riickert, on a number of other copepods (Cyclops, Canthocamptus\ 

 in which rings are not formed, since the splitting of the primary 

 chromatin-rods is complete. The origin of the tetrads has here been 

 traced with especial care in Cyclops strenuus, by Riickert ('94), whose 

 observations, confirmed by Hacker, are quite as convincing as those 



a 



Fig. 124. Diagrams of various modes of tetrad-formation. [HACKER.] 

 a. Common starting-point, a double spireme-thread in the germinal vesicle ; d. common re- 

 sult, the typical tetrads ; b. c. intermediate stages : at the left the ring-formation (as in Diaptomus, 

 Gryllotalpa, Heterocope) ; middle series, complete splitting of the rods (as in Cyclops according to 

 Riickert, and in Canthocamptus) ; at the right by breaking of the V-shaped rods (as in Cyclops 

 strenuus, according to Hacker. 



of Brauer on Ascaris, though they led to a diametrically opposite 

 result. 



The normal number of chromosomes is here twenty-two. In the 

 germinal vesicle arise eleven threads, which split lengthwise (Fig. 123), 

 and finally shorten to form double rods, manifestly equivalent to the 

 closed rings of Diaptomus. Each of these now segments transversely 



