SOMATIC CHARACTERS AND CHROMATIN ROD-LENGTHS. 109 



throughout their entire length, and errors in draughtsmanship have been 

 minimised by drawing each individual chromosome many times; the 

 measurements given should therefore represent the true dimensions with as 

 great accuracy as can be obtained with the means now at our disposal. 



Spermatogenesis. 



The testes are two ovoid bodies lying in the middle of the abdomen 

 dorsally to the alimentary canal ; they are composed of tubular follicles 

 tapering towards the ends and divided into numerous tracts and cysts. The 

 primary and secondary spermatogonia lie at the extreme anterior end of each 

 follicle, and next to these are large areas occupied by cells undergoing the 

 growth-period ; no resting stage occurs between the maturation mitoses, and 

 the next portion of the follicle is accordingly occupied by both primary and 

 secondary spermatocyte divisions. The posterior end contains spermatids 

 undergoing transformation to unripe and ripe spermatozoa. As I have 

 already pointed out in the case of S. viridulus, all cells in one cyst are not at 

 precisely the same stage of development, and in a transverse section mitotic 

 figures and resting stages can be seen lying side b}^ side. 



The primary spermatogonia are arranged in a cluster at the extreme 

 anterior end of the follicle, whereas the secondary spermatogonia, more 



posteriori}- placed, are in greater numbers 

 and appear to be without definite arrange- 

 ment in the cyst. The chromatin during 

 the resting stages is disposed in granules 

 upon linin threads, and the nucleus is appa- 

 rently a complete reticulum. In the pro- 

 phase of division this network breaks into 

 numerous filaments, which shorten and con- 

 dense until seventeen compact and smooth 

 chromosomes are seen lying in the equatorial 

 plane ; these are divisible into eight pairs 

 Fig. 1.— Sjjeruiato^oalal meta^jLiase. and one odd chromosome, which corresponds 



with the monosome oF Davis and the hetero- 

 tropic, accessory, and X chromosome of other writers. 



The sixteen ordinary chromosomes are graded in length and individually 

 composed of two equal rods, of which one passes to each pole in the subse- 

 quent anaphase : the plane of cleavage is invariably parallel to the major 

 axes of these rods, which appear to be indivisible units. The diameter of the 

 ordinary rods is constant, whereas that of the odd chromosome is greater and 

 varies throughout its length ; the latter is thus easily distinguishable from 



