610 NATHAN FASTEN 



It is thus seen that here we have to deal with a side by side 

 conjugation of the chromosomes (parasynapsis), and the synaptic 

 and pachytene stages are very similar to those found in Euschistus 

 (Montgomery '11), in Batrachoseps (Janssens '05), in Tomopteris, 

 Salamandra, Myxine, Spinax (the Schreiners '06 a and b), and 

 in Cyclops (Lerat '05). 



Stage F (figs. 14-15) : Diplotene (strepsineme) stage. Here 

 the fused chromosomes of the pachytene threads unravel. Each 

 geminus, which consists of two single chromosomal threads in 

 parallel union, becomes split longitudinally, along the line of 

 their original union. These threads then open up at one end 

 while remaining attached at the other, taking the shapes of 8, 

 V and U (figs. 14-15). There is no evidence that the arms of 

 the geminus twist about each other as in Batrachoseps or in 

 Euschistus. 



Stage G (figs. 14-20) : Postdiplotene stage. The individual 

 arms of each geminus open up still more completely at one end 

 while remaining partially fused at the other end. Each arm then 

 splits longitudinally (the second longitudinal split), producing 

 four thin arms which stain slightly (figs. 14-17). The arms then 

 diverge, forming X's and 8's (figs. 17-20). The centrally con- 

 stricted point of these figures is the place where the four arms 

 are in fusion. This is particularly well seen in figure 17, from 

 a smear preparation. Wilson ('12), in Oncopeltus and Lygaeus 

 found a diffuse stage between steps F and G, in which the lepto- 

 tene threads disappear from view, giving rise to vague, coarse and 

 lightly staining structures within the nucleus. In the species 

 of Cambarus under discussion no such diffuse stage was found. 



Stage H (figs. 20-22): Tetrad formation. The tetrads are 

 formed in the following way: the four arms of each geminus 

 shorten and thicken. Their central point of fusion partially 

 disappears, giving the geminus the appearance of being traversed 

 by a longitudinal and a transverse split (figs. 20-21). The four 

 thin threads then condense into four round chromosomes, form- 

 ing tetrads (figs. 21-22). Each pair of chromosomes next unites 

 to form a bivalent type and these are attached to each other 

 by thin linin strands (figs. 22-23). This is the final step in the 



