282 THOS. H. MONTGOMERY, 



The outermost layer of the testicular sheath is composed of large, 

 irregularly branched, much flattened cells, which do not form a closed 

 layer, but which seem to touch one another only by the ends of their 

 processes. They may be interpreted as nurse or yolk cells, providers 

 of nutritive or yolk substances for the spermatogonia, as the following 

 description will show (Figs. 233, 234, 238 Yk. G). 



Each nurse cell is much flattened, with a very delicate, lightly- 

 staining cytoplasmic structure, and an exceedingly delicate cell mem- 

 brane. Surface views show them to be always irregular in outline 

 (Figs. 235 — 237). The shape varies, in the same testis, from an ir- 

 regularly spherical, in which case there is but little yolk substance 

 in the cell, to an exceedingly irregular lobular, branching form, 

 characteristic of those which contain the most yolk substance. Hence 

 there would seem to be a direct physiological connection between the 

 form of the cell and the amount of yolk substance contained in it: 

 the cell is thinnest and most expanded when it has the most yolk. 

 Since at one stage the form is not irregularly lobular, these cells 

 must be considered amoeboid with the power of changing their form, 

 possibly even they are not fixed in position but have the power of 

 migrating about on the surface of the testis. 



The nuclei of these cells are large with proportionately less 

 chromatin those of the muscle cells. They are either disc-shaped, 

 round or oval in outline, with one or two small nucleoli (Figs. 236, 

 237); or they are thicker, irregular in form, with a considerable 

 number of irregular true nucleoli, the latter being often lobular 

 (Fig. 235). The former type of nucleus is the one usually found. 

 The irregular nuclei seem to be limited to those cells which have 

 only a small amount or no yolk; and the two forms of nuclei seem 

 to correspond to the two types of cells mentioned above. Though 

 these nuclei are so often irregularly constricted, I could find no case 

 which would speak positively for amitosis; and in one case I found 

 a typical mitosis (Fig. 238). 



With the saffranine-violet stain the chromatin of these nurse cell 

 nuclei stains usually violet, as is typical for chromatin in the rest 

 stage. But sometimes (in the same section) it stains red throughout 

 though there is no evidence of any stage of mitosis, and such 

 nuclei might be considered degenerative; comparable to them then 

 would be the similarly staining follicular testis cells in Amphibia. 



The yolk substance {N. Gl Figs. 233, 236—240) in these cells is 

 usually in the form of small spherules in the cytoplasm. These vary 



