THE SPERMATOGENESIS OF DOMESTIC MAMMALS. "369 
probably cast off with the main portion of the cytoplasm out of the body of 
the spermatozoon. 
VIII. Mitochondria. 
The mitochondria appear in small numbers throughout the cytoplasm in 
the growth stage (Figs. 23, 24) and are recognisable in all the succeeding 
stages up to the development of the spermatozoa. 
In the prophase of the first reduction division they are still found scattered 
throughout the cytoplasm. In the metaphase of the first reduction division 
the mitochondria remain undivided and lie outside the spindle (Figs. 38, 39), 
but after the division they seem to be approximately equally distributed in 
the daughter cells. In the resting stage of the secondary spermatocyte the 
mitochondrial granulations are also seen scattered all over the cell body, but 
a portion of them group together and lie near the idiozome which now 
appears in the cell (Figs. 49, 50). Their behavior in the second reduction 
division is similar to that in the first division. 
In the spermatid most of the mitochondria group together and lie at 
the posterior part of the cell body while the rest of them still lie scattered 
in the cell body (Fig. 62). The fate and behavior of the mitochondria during 
the development of the spermatozoa are very significant. Simultaneously with 
the division of the centrosome the mitochondria which lie at the posterior 
part of the cell body commence to fuse together and form a mass 
which is similar to the “ Nebenkern” in insects (Figs. 62, 69). But this 
mitochondrial mass is very indistinct, having no sharp contour as compared 
with those seen in the lower animals (Fig. 69). Its location at this stage is 
the posterior part of the cell (Figs. 62, 69), where it finally encloses the 
axial filament and the centrosomes (Fig. 69), and thus comes to occupy the 
middle piece of the spermatozoa (Fig. 73). During these changes the granula- 
tions gradually disappear and it becomes more homogeneous (Figs. 74, 75). 
A small portion of the mitochondria which lie scattered in the cell body in 
the early stage of the spermatid is probably cast out of the body of the 
spermatozoon (Fig. 73). 
