192 



REDUCTION OF THE CHROMOSOMES 



vom Rath. Flemming observed the double V-shaped loops in 1887, 

 and also the tetrads derived from them, but regarded the latter as 

 "anomalies." Vom Rath ('93) subsequently found that the double 

 V's break at the apex, and that the four rods thus formed then draw- 

 together to form four spheres grouped in a tetrad precisely like 

 those of the arthropods. Still later ('95, i) the same observer traced 

 a nearly similar process in the frog ; but in this case the four ele- 



Fig. 96. — Germinal vesicles of various eg^s, showing chromosomes, tetrads, and nucleoli. 



A. A copepod (Hcterocope) showing eight of the sixteen ring-shaped tetrads and the nucleo- 

 lus. [RUCKERT.] 



B. Later stape of tlie same, condensation and segmentation of the rings. [ROCKERT.] 



C. "Cyclops sdeniiif'," illustrating Hacker's account of the tetrad-formation from elongate 

 double rods; a group of " accessory nucleoli." [Hackf.r] 



D. (jerminal vesicle of an annelid {Ophrvofi-ocha) showing nucleolus and four chromosomes. 



[K(_)RSCHF,l/r.] 



ments appear to remain for a short time united to form a ring before 

 breaking up into separate spheres. 



To sum up : The researches of Riickert, Hacker, and vom Rath, 

 on insects, Crustacea, and amphibia have all led to the same result. 

 However the tetrad-formation may differ in matter of detail, it is in 

 all these forms the same in principle. Each primary chromatin-rod 

 has the value of a bivalent chromosome ; i.e. two chromosomes 

 joined end to end, ab. By a longitudinal division a ring or double 



