126 DU. J. STEPHENSON ON THE MORPHOLOGY, CLASSIFICATION, 



segm. X. Each is a short, thick cylinder, with a soft, white 

 lobulated surface and a central canal opening on the surface in 

 groove 10/11. 



The outermost coat is by far the most bulky ; it is composed 

 of large cells (jjrost.) with distinct outlines, the cytoplasm of 

 which stains moderately with haematoxylin. The cell substance 

 is in general granular, the nuclei (mtc.) are small, homogeneous, 

 of various shapes, often peripherally situated, and do not stain 

 much more deeply than the cytoplasm. In one specimen there 

 are numerous vacuoles [x.) in the cells, each with a small, round, 

 homogeneous, lightly staining mass in the centre. 



In thickness this layer is 120 /x or ' more. The cells do not 

 pierce the muscular and connective-tissue layers to join the 

 epithelial lining of the prostatic lumen; some cells are sessile on 

 the muscular layer, others have their rounded inner ends slightly 

 separated from it (c/. fig. .3). There can, I think, be no real 

 doubt that the layer of large cells is not a bursting of the lining 

 epithelium through the muscular coat. 



Outside this layer there is often to be seen a peritoneal coat 

 as a very thin blue-staining membrane — a mere line — with here 

 and there more or less flattened nuclei, often -apparently 

 degenerate. In some cases the membrane spans the angle 

 between the outer ends of neighbouring cells. Sometimes a few 

 muscular fibres may be seen outside the layer of large cells. 

 The peritoneal layer may also carry blood-vessels of considerable 

 size, some branches of which penetrate the layer of large cells. 



From what are the cells of this layer derived ? The presence 

 of a peritoneal layer surrounding them would seem to preclude 

 the possibility of their being themselves peritoneal in origin, as I 

 at first supposed ; while the fact that they are totally cut off 

 from the lining epithelium — separated from it by the muscular 

 layer — seems to shut out the possibility of referring them to the 

 epithelial layer. 



I believe that they are peritoneal in origin, however ; the fact 

 tliat they are covered in places by a flattened peritoneal epi- 

 thelium does not necessarily negative this view. A peritoneal 

 membrane persists over the layer of large cells in the same way 

 that such a membrane persists over the lobes of the testes in 

 species of Lumhricillus : the testes are proliferations of the 

 coelomic epithelium, which in this genus (as in Ocnerodrilus) are 

 .covered in at first by a delicate peritoneal membrane. 



(4) The Vas Deferens. 



The vas deferens descends from the testis sac to the ventral 

 body-wall, and in doing so forms a number of coils on both sides 

 of septum 9/10. In this part of its course its diameter is about 

 40 /x ; its lining epithelium consists of cells staining pink with 

 eosin, which almost occlude the lumen and whose nuclei are 

 situated below the middle of their height ; Ifc possesses also a 



