SPERMATOGENESIS OF EUSCHISTUS 747 
narrower edge it more frequently seems to be a solid mass of chro- 
matin. The nucleus thus remains vesicular. 
Mature sperm heads of a particular testicular follicle vary 
somewhat in length, as shown by me before (10a). But this 
appears to be a case of continuous variation, and those of the 
same follicle do not constitute two or more constant size groups, 
nor do they exhibit polymorphism. 
No evidence was found of the casting off of any substances by 
the sperm, such as has been described in mammals and in Myxine. 
Further, all the sperm of a follicle appear to develop, and when 
degeneration phenomena are found (such as marked vacuolization 
of the sperm heads) they usually include all the sperm of a cyst. 
There is no evidence that particular sperm degenerate, nor any 
particular proportion of them. 
During their earlier stages the spermatozoa are arranged irreg- 
ularly within their cysts. But after they have lengthened, as 
in the stage of fig. 126, all those of a cyst become grouped into 
bundles with the heads close together and directed towards the 
vas deferens, while their tails point in the opposite direction. 
After the heads have grown much longer, as in the stage of fig. 146 
or a little earlier, all the spermatozoa of a bundle have the pointed 
ends of their heads imbedded in the cytoplasm of one of the larger 
follicular cells that form the thickened epithelial lining of the dis- 
tal end of the testis (compare the text figure of my paper, ’10a); 
but this attachment, which may denote a feeding process, becomes 
broken before the sperm arrive in the vas deferens. 
B. Discussion 
In his classical memoir of 1883 the lamented Eduard Van Bene- 
den gave the first statement of the theory of the individuality 
of the chromosomes, and at the same time determined that 
the mature germ cells contain half the normal number of chromo- 
somes, two conclusions that are of the greatest importance. Since 
that time much attention has been given to the mechanism of 
these processes, and the interest in them never seemed more 
intense than at the present time. 
