628 The American Naturalst. Uuly, 
ceedingly variable, so that exact times cannot be given. The gen- 
eral appearance is illustrated by Fig. 16. from Kölliker. The 
vesicle figured was 4.4 mm. in length ; the envelopes of the ovum 
are not shown, though they were still present; at the upper pole 
is the small embryonic shield, corresponding in position to the 
inner mass; it is marked out by the greater thickness of the 
walls of the vesicle; the developing second layer extends over 
more than half the vesicle, reaching to the line ge. 
The following is a summary of Ed. van Beneden’s description, 
2, 185-200, of the blastodermic vesicle of a rabbit at 6 days, 1% 
hours after coitus. The vesicle measured 3.2 mm. in diameter ; 
it was nearly spherical; the wall of one hemisphere consisted of 
one layer of cells; the other hemisphere had two layers of cells, 
and besides in its central portion a third layer intervening between 
the other two. The area with three layers van Beneden desig- 
nates as the tache embryonnaire ; it showed no trace of the primi- 
tive streak ; it was oval in outline, and had one point, which the 
author identifies as Hensen’s knot, where the layers adhere to- 
gether closely. Transverse sections show that the outermost 
layer of cells is a low cylinder epithelium, which at the edge of 
the area passes into a thin epithelium quite abruptly ; it corre- 
sponds to Rauber’s “ Deckschicht,’ and has been said by him to 
flatten out and disappear, leaving the cells underneath as the outer 
layer of the embryonic vesicle in later stages, compare the fol- 
lowing paragraph. The cells of the innermost layer are thin and 
wide; they are called the hypoblast (entoderm) by van Beneden ; 
the cells themselves have round nuclei, around each of which is 
accumulated a court of granular protoplasm ; the adjacent courts 
are connected by a coarse meshwork of protoplasmic threads ; 
treatment with nitrate of silver brings out the cell boundaries, 
and divides the reticulum into polygonal areas. The cells of 
the present outermost layer have distinct boundaries and contain 
granules, and long bacilliform bodies, which van Beneden saw 
also in the fresh specimens, and found to be constant appear- 
ances. Similar bodies are found in the germinal vesicles of 
sheep, and are held by Bonnet, 70, to be derived from the uterine 
milk; the rabbit is not known to have uterine milk. The histo- 
