244 



INTRODUCTION TO CYTOLOGY 



these may arrange themselves with reference to one another — in the form 

 of simple or compound rods, crosses, and rings — their distribution to the 

 daughter nuclei, as well as the manner of their origin, is very difficult to 

 follow with certainty. The accompanying diagrams will serve to illus- 

 trate the more common modes of behavior described for chromosome 

 tetrads, which are found chiefly in the cells of animals. 



Figure 95, D represents an exceptional method of tetrad formation 

 described by Henking (1891) for Pyrrochoris and by Korschelt (1895) for 

 Ophryotrocha. The continuous spireme segments to form the diploid 

 number of chromosomes, 1 which then split longitudinally and shorten. 



D 



E 



>J> 



S" 



Fig. 95. — Reduction with chromosome tetrads. 



D, in Pyrrochoris (Henking) and Ophryotrocha (Korschelt.) E, in certain copepods 

 (Riickert, Haecker, and vom Rath.) F, in Anasa and Allolobophora (Paulmier; Foot and 

 Strobell). 



No conjugation occurs until the metaphase, when the split chromosomes 

 come together end-to-end, forming tetrads. They at once separate in 

 the anaphase, bringing about reduction. In the second mitosis they 

 divide along the original split, so that each of the four resulting nuclei 

 receives the haploid number of chromosomes, two of the nuclei thus 

 differing from the other two as the result of the separation of entire 

 (though secondarily split) chromosomes at the first mitosis. According 

 to Goldschmidt (1905), the chromosomes of Zoogonus minis, after thus 

 undergoing no prophasic conjugation, divide longitudinally at the first 



1 For the sake of uniformity and clearness the diploid number is represented as 6 

 in all of these diagrams. 



