1886.] OVUM OF LEPIDOSIREN. 279 
IV. The Follicular Epithelium and the Formation of Yolk. 
In the youngest ova (Plate XXIX. fig. 8) the protoplasm is dense 
and solid, staining deeply with borax carmine; the uniform appear- 
ance of the egg-contents indicates that no formation of yolk has at 
present commenced. 
In the next stage, where the zona radiata is well developed 
(Plate XXVIII. fig. 1), the protoplasm of the ovum is less dense and 
has acquired here and there a reticulate arrangement, which is well 
shown in the figure referred to. 
In ova slightly more mature (Plate XVIII. fig. 2) the formation of 
the yolk is in active progress, though for the present confined to 
limited areas of the egg-protoplasm. None of my sections display 
any ova which show the first beginning of yolk-formation. In 
Plate XXVIII. fig. 2, it will be noted that the yolk appears in 
patches usually spherical in shape and larger or smaller; the yolk 
has the form of minute spherules and aggregations of spherules, which 
are easily distinguishable from the surrounding protoplasm, which is 
also granular, by their more coarsely granular appearance. They 
have also been stained much more darkly by the borax carmine. 
The yolk-spherules invariably make their appearance in the inter- 
spaces between the reticulations of the egg-protoplasm. There is 
thus no doubt that the yolk is actually formed in the interior of 
the ovum at the expense of the egg-protoplasm ; the reticulation of 
the egg-protoplasm, invisible in earlier stages, but completed before 
the appearance of yolk, is probably a preparation for the formation 
of the latter, which is received when formed into the interspaces 
between the protoplasmic strands. 
Ova of the next stage (Plate XXVIII. fig. 4) show the yolk- 
spherules well developed and filling up the entire ovum; the sphe- 
rules themselves are small and of varying size ; occasionally numbers 
of yolk-spherules had run together to form irregular shaped masses ; 
these yolk-masses, owing to their size and impenetrability, were in- 
variably left unstained. 
The yolk at this period entirely fills the ovum, and is spread 
throughout it in a perfectly uniform fashion ; there was no indication 
of any peripheral layer free, or nearly free, from yolk. 
The ovum displayed in Plate XXVIII. fig. 4 is remarkable for the 
fact that the yolk is distinctly differentiated into two layers—a thin 
peripheral layer, and a central mass ; the boundary between the two 
was perfectly distinct, there being an absolute break, a narrow line 
perfectly free from yolk-spherules ; the outer layer was also rendered 
more conspicuous by the lighter staining of the reagent. I have 
noticed several ova among my sections which displayed this curious 
differentiation. 
Throughout its whole development the ovum is surrounded by a 
single layer of follicular epithelium-cells ; these cells are flattened as 
in the Amphibia, and have a large deeply staining nucleus. In most 
aii a membrana propria folliculi could be detected outside these 
cells. 
