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H. E. Jordan 
fate was not determined. They probably arise as the result of nuclear 
multiplication unaccompanied by cytoplasmic fission (rather than fusion) 
and may eventually separate as described by Guyer for chicken — since 
no megalosperms were observed in my material. 
VIII. Sertoli Cells (trophocytes). 
When the spermatid has attained a development of about the stage 
represented in fig. 67, it attaches itself to a Sertoh cell. Usually 4 are 
attached to a cell. Sometimes, however, as many as 8 combine to form 
a single spermatoblast. Sertoli cells seem unusually abundant in the 
opossum. The spermatids undergo their later development attached to 
the trophocytes, and again free themselves at about the stage represented 
in fig. 81. The Sertoli cells are tall columnar cells, tapering distally, 
with broad basal portions containing large vesicular nuclei eacli with 
large central plasmosome. 
The most significant fact regarding the Sertoli cells in this Connec- 
tion is the large quantitv of deutoplasmic material they eontain. T h is 
is in the form of larger and smaller spheres, stained deeply by the osmic 
acid of the Flemmexg’s fluid. I have been unable to identify definitely 
characteristic mitochondria (due to the abundance of fat) in fliese cells, 
thougli they may be present in small quantity. The larger spherules of 
fat are found only in the basal portions of the cells. Centralwards the 
fat is more finely divided and stains less intensely, a condition exactly 
parallel to tliat recently described by Haxes (1909) for the testicle of 
the pig and certain other mammals. 
The large amount of deutoplasmic material in the trophocytes un- 
doubtedly furnishes the energy for the growth and metamorpliosis of 
the spermatids into sperm, and probably also for the increase in the amount 
of cliondriosomes in the spermatids. 
IX. Formation of the Spermatozoa. 
Folio wing a brief resting pliase during which the spermatid remains 
small (fig. 61) with small nucleus (with achromatic reticulum containing 
several karyosomes) and closely applied oblong centrosphere (idiozome) 
there ensues the first stage in the process of metamorphosis to a Sperma- 
tozoon. This stage is characterized by enlargement of the nucleus, the 
disappearance of the karyosomes coincident with the appearance of a central 
