158 THE CHROMOSOMES 
each other, beginning at the end where they first 
approached each other. The details of the union 
of the threads are further shown in f, g, h. As they 
unite they contract until they are in the form of a 
thicker thread, as seen in 7, where the process of 
fusion has progressed as far as the middle of the 
nucleus. Later, 7, 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 Maréchal 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 
