76 ROBERT H. BO WEN. 



istic vesicles, the intervening substance being simultaneously spun 

 out into thin connecting threads. The result (Fig 27) is that 

 each of the sheaths becomes spun out into a long, delicate thread 

 along the course of which these vesicles are scattered as bleb-like 

 swellings. The structure of these swellings is very clearly demon- 

 strated by cross-sections of the tail. In such sections (Fig. 28) 

 the sheaths themselves appear as small dots intensely stained 

 with crystal violet, while the vesicles appear as masses of non- 

 staining material enclosed by a peripheral layer which stains like 

 the portions of the sheaths which intervene between two adjacent 

 vesicles. It is as if the thread-like sheaths were inflated at inter- 

 vals to form little pockets in which is placed a small mass of 

 some material staining quite differently from the substance proper 

 of the threads. My impression is that these vesicles are eventu- 

 ally smoothed out, their substance being perhaps distributed 

 evenly along the thread-like mitochondrial sheaths. Needless 

 to add,, these sheaths are not broken down as stated by Holmgren 

 and Vejdovsky. (See Bowen ('22)). It is also clear that it is 

 these vesicles which Vejdovsky has confused as the " fat drop- 

 lets " derived from the breaking down of the mitochondria as a 

 whole. Obviously, their real nature is entirely different. 



The exact origin of these vesicles has not been satisfactorily 

 made out, and the relation of their structure to that of the earlier 

 stages is therefore uncertain. It is possible, however, that the 

 central substance of the nebenkern derivatives tends to collect at 

 intervals along the course of the sheaths, thus forming the bleb- 

 like swellings, while in the intervening lengths, from which the 

 central substance is withdrawn, the sheath is correspondingly 

 thinned out (Fig. 27). Possibly such accumulations of material 

 play some role in the spinning out of the sheaths. The ultimate 

 fate of this central substance is unknown; possibly it comes 

 finally to form a delicate core for each of the thread-like mito- 

 chondrial sheaths. Thus is completed the long and complicated 

 differentiation of the spermatid nebenkern. 



CONCLUSION. 



One of the most interesting results of this study has been to 

 reveal the nebenkern as the seat of an unexpectedly complicated 



