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, w^hich, by further contraction, 

 reaches the condition shown in I, 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 

 w^hich 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- 

 phcation 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 6, c, when the separate 

 thin threads begin to make their appearance, and 

 take parallel courses, d (leptotene stage). These 



