THE CHROMOSOMES 125 



each other, beginning at the end where they first 

 approached each other. The details of the union 

 of the threads are further shown in /, g, h. As they 

 unite they contract until they are in the form of a 

 thicker thread, as seen in i, where the process of 

 fusion has progressed as far as the middle of the 

 nucleus. Later, j, the threads become fused through- 

 out their length (pachytene stage). Still later the 

 thick threads begin to show a longitudinal split 

 (diplotene stage), and cross connections, uniting the 

 halves of the threads, appear in different places. 

 The threads thicken until finally a stage is reached 

 like that shown in k, which, by further contraction, 

 reaches the condition shown in /, a stage preparatory 

 to the first maturation division. The threads of 

 each pair, in all the stages of the latter part of the 

 diplotene stage, are much twisted around each 

 other; they are now so thick that they show the 

 twisted condition very plainly. 



The egg undergoes a series of changes during its 

 maturation which parallels those of the sperm, and 

 which leads also to the reduction in the number of the 

 chromosomes to half of the full number. The eggs 

 of a shark (Pristiurus melanostomus) have been 

 described by Marechal as passing through the 

 following stages. At the end of the period of multi- 

 plication the eggs pass into a resting stage (Fig. 44, a) 

 in which the chromatin appears as a delicate reticu- 

 lum. A later stage is shown in b, c, when the separate 

 thin threads begin to make their appearance, and 

 take parallel courses, d (leptotene stage). These 



