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. I), 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. Tliey 

 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 impenetrabihty, 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 ray sections which displayed this curious 

 diflFerentiation. 



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 

 cases a membrana propria folliculi could be detected outside these 

 cells. 



