The spermatogenesis of Peripatus (Peripatopsis) balfouri. 281 



or less of a lumen; between these two zones lie the spermatocytes 

 in stages leading up to the rest stage. Hence on a transverse section 

 of the testis one may find any or all of these stages; and the dis- 

 tribution of the several stages in concentric zones is the same at 

 the proximal end as in other portions of the testis. But few sperm- 

 atids are to be found in the testis: this is because at about the time 

 of the beginning of the maturation divisions the spermatocytes round 

 ofif, and thereby come (for the most part) to lie free in the axial 

 lumen of the testis; and the second spermatocytes are generally pro- 

 pelled (probably by contractions of the muscular sheath of the testis) 

 into the lumen of the seminal vesicle. 



a) The Sheath of the Testis. 



The envelope of the testis is composed of two inner muscle layers, 

 and of an outer layer of nurse cells (Figs. 233, 234, 238, Plate 23). 



The muscle cells (M. C) are arranged in an outer and a inner 

 layer, separated by a narrow irregular space transversed by delicate 

 non-nucleated fibrils (interstitial tissue), and filled with a structure- 

 less fluid (haemolymph). 



In the outer muscular layer the fibrils run transversely, diagonally 

 and longitudinally, but all parallel to the surface of the testis; they 

 can be best seen on paratangential sections (Figs. 241, 242). Their 

 nuclei (M.N) are flattened, large, more or less elhptical in surface 

 outline, elongate when seen on cross section of the testis ; they are 

 rich in chromatin and contain one or two true nucleoli each. Each 

 nucleus is imbedded in a finely granular mass of cytoplasm, from the 

 periphery of which the contractile fibrils take their origin. Each cell 

 has a considerable number of fibrils arranged in one plane. Each 

 fibril is smooth, and appears almost homogeneous with a darker peri- 

 phery and lighter core; there is no evidence of transverse striation 

 even with intense iron haematoxyline staining. The fibrils are separated 

 from one another by narrow spaces. To each cell there belong, ac- 

 cordingly, a number of fibrils which proximally are parallel to one 

 another but distally diverge. 



In the inner muscle layer the fibrils are also in a single layer, 

 mainly transverse in their course, and usually thicker than those in 

 the outer layer. Their fibres have the structure above described, but 

 distally diminish in diameter by the gradual disappearance of the clearer 

 core. I could find no clear cases of anastomoses or of branching of 

 these fibres, 



