SPERMATOGENESIS OF THE HORSE. 303 



as in the case of the pig (Figs. 74, 75, 77 and 84). The chroma- 

 toid body is, in rare cases however, also seen on the filament 

 and in such cases apparently fused with the posterior centro- 

 some (Figs. 71 and 76). It is invariably sloughed off before the 

 spermatozoon is fully developed (Figs. 77 and 86). 



Shortly after the centrosome divides the nucleus begins to 

 elongate and at the same time migrates toward one end of the 

 cell, so that soon practically all of the cytoplasm is found at the 

 posterior end of the developing sperm (Figs. 67-75). As the 

 acrosome-end of the nucleus comes in contact with the cell-wall 

 no break in the latter is ever noticeable, and the apparent back- 

 ward pull exerted by the mass of cytoplasm causes the cell- 

 wall to become closely applied to the nuclear wall where it 

 undoubtedly persists as an additional covering of the sperm- 

 head (Figs. 73-79). This supposition that the cell-wall forms 

 an additional covering of the sperm-head is based on two obser- 

 vations; firstly, there is no evidence that the head penetrates 

 the cell-wall, and secondly, the covering of the sperm-head is 

 much thicker after the entire mass of cytoplasm lies at its 

 posterior end (Figs. 73-79). This fact gives one the impression 

 that the distinctly noticeable change in the thickness of the 

 head covering is brought about by the fusion of the two walls. 

 It is also obvious that the cell- wall is not entirely consumed in 

 forming the external covering of the head of the sperm, for it 

 can always be seen surrounding the anterior portion of the axial 

 filament and extending far down into the mass of cytoplasm which 

 is apparently squeezed out of it and about to be thrown off 

 (Figs. 77-79). In the final stages it becomes closely applied to 

 the axial filament and one may safely conclude that the axial 

 envelope is at least partly formed by the portion of the cell-wall 

 extending down from the head (Figs. 77, 79 and 85). This same 

 condition was found to exist in the pig. 



When the developing sperms reach the stage represented in 

 Fig- 73 they become attached in clusters to the large nurse cells. 

 As the sperms develop the cytoplasmic mass of the nurse cells 

 decreases. Just as the mass of cytoplasm is being thrown off by 

 the developing spermatozoa, the latter leave the nurse cells and 

 become embedded in the layer of cytoplasm composed of the 



