ON SPERMATOGENESIS IN THE RAT. 
345 
In a section stained with haematoxylin tubules are seen, cut 
more or less transversely, and presenting different appearances 
according to the stage of development of their contents. 
Since the process of development of spermatozoa is a con- 
tinuous one, it would be possible to take any one stage as a 
starting-point for description ; but it is, on the whole, more 
convenient to start with the stage represented in fig. 1, in 
which a crop of spermatozoa is fully formed and ready to leave 
the tubule. 
A tubule in this stage (fig. 1) consists of four layers of 
seminal elements, with a basement membrane formed of 
flattened cells, and these four layers correspond to four genera- 
tions. 
The most external layer immediately within the basement 
membrane consists of cells, the nuclei of which are all in the 
resting condition. 
Of these nuclei there are three kinds represented in fig. 1 
corresponding to three classes of cells. 
(1) There are large pale nuclei, the diameter of which is 
about 18 /a, each containing a distinct round nucleolus, and 
being bounded by a definite nuclear membrane (fig. 1, e). In 
this stage some of these nuclei are seen resting upon the base- 
ment membrane, while others are seen extending between the 
cells of the second layer, and here and there one is found 
among the cells of the third layer. These nuclei belong to 
supporting cells, each of which serves for the support of a 
group of spermatozoa during their development. The proto- 
plasm of these cells forms a sort of network upon the base- 
ment membrane, in the meshes of which the other cells of the 
outer laver are contained, while from the inner extremitv of 
the nucleus a protoplasmic process extends towards the sper- 
matozoa in the lumen of the tube. These cells appear to be 
constant among Vertebrata; they have received various names 
from different investigators, and have had very different func- 
tions assigned to them. 
(2) Besides these supporting cells there are a considerable 
number of small cells containing oval nuclei, which stain 
