10 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



size and come to resemble eggs, and, because the supply of nourishment 

 to be obtaiued by some cells is more than that accessible to others, there 

 arise two types of cells, which, as long as this different condition endures, 

 exhibit considerable differences in their behavior. When I first observed 

 these two types, their apparently marked differences at certain stages 

 led me to believe that they must have arisen from cells which were 

 radically different in preceding generations, but upon more careful study 

 this was found not to be the case. There being but one type of sperma- 

 togonium, I am led to believe, from extensive observations, that the 

 differentiation arises as a secondary characteristic in the spermatocyte 

 stages alone. The late spermatogonia and the early spermatocytes ex- 

 hibit no such divergence whatever. Study of later stages also reveals 

 the fact that the very evident differences of the two kinds of cells are not 

 so important as was at first supposed. For in their later behavior, in 

 the second spermatocyte and in the spermatid, the cells of the two types 

 again exhibit practically identical phenomena. It is very interesting to 

 note in this connection that the slight differences in the environmental 

 conditions characteristic of the large and small types of spermatocytes at 

 the earlier stages, no longer exist in the later ones. The relation of the 

 cells to their food supply is now practically identical in the two types, 

 and as a consequence they behave in a manner as nearly alike as is pos- 

 sible in view of the marked difference in size and the slight variation in 

 structure. From these facts it seems to me very improbable that these 

 two types of cells develop into essentially different kinds of spermatozoa. 



2. Spermatogonia and Early Spermatocytes. 



The spermatogonia of Scolopendra heros are rather small cells, which 

 in the well-developed material principally used in these observations are 

 confined exclusively to the extreme periphery of the testicular follicles 

 (Fig. 1 ; Plate 8, Fig. 123). In shape they are oval, the long diameter, 

 which is several times greater than the short, being parallel to the long 

 axis of the follicle. The cells of the earlier stages are often in such very 

 close contact with the fibrous sheath of the testis that it is difficult 

 to study them carefully. However, occasionally there are to be found 

 groups of a dozen or more (Fig. 123) which project out into the mass 

 of young spermatocytes, and it was mainly upon these that the observa- 

 tions of the earlier stages were made. 



In what may be called the resting stage of the spermatogonia (Plate 2, 

 Fig. 2) the nucleus,, which fills a large part of the cell, is an oval body sur- 



