DEVELOPMENT OF GERMINAL LAYERS IN MAMMALS, 42] 
who, in describing the appearance of the blastodermic cavity, 
attributes it not, as van Beneden does, to the separation of 
the inner mass from an outer layer, but to the appearance of 
a cavity amidst a number of similar elements. He says, 
“ Die Furchungshohle bildet sich durch einen langsam wach- 
senden Serum-Erguss im Innern des vollstanudig in gleich- 
grosse Furchungskugeln zerlegten Dotters, an excentrisch 
gelegnen Stelle der ganzen Erkugel”’ (88, p. 104). 
After the appearance of the blastodermic cavity the blasto- 
cyst is rapidly expanded ; between the seventy-fifth and ninety- 
fourth hours it becomes doubled in size, and at the latter 
period the wall of the vesicle, which measures 0°28 mm. in 
diameter, consists in the greater part of its extent of a single 
layer of flattened cells; the remainder of the wall is formed 
by a mass of cells, and of these, those situated most externally 
are apparently, according to van Beneden, similar in appear- 
ance to the flat cells which form the greater part of the wall 
of the vesicle, with which they are said to be directly con- 
tinuous; and thus has arisen the description of a vesicle with 
a wall formed by a layer of flattened cells, and containing an 
inner mass which is adherent to a portion of the vesicle wall. 
It is admitted, however, that the inner mass is intimately 
attached to the outer layer, and upon this point Rauber speaks 
very clearly. ‘‘ Nur an einer Stelle ist die so gebildete Keim- 
blase nicht einschichtig und sind die Furchungskugeln nicht 
platz geworden, sondern eine dunkle Gruppe von Furchungs- 
kugeln ragt daselbt mit convexer Oberflache in das Innere 
der Keimblase vor, eine biconvex Scheibe dastellend.” 
“Diese Scheibe liegt nicht etwas lose und verschiebar der 
wand der Keimblase an, sie andert nicht ihren Platz an der 
wand der letzteren, wenn das El bewegt wird, sondern sie 
bildet einen integrirenden Theil der Keimblasenwand selbst 
und geht mit ihrer Urmundrandung in den einschichtigen 
Theil der Keimblase tuber” (38, p. 104). 
It is admitted that the inner layer of the inner mass is the 
germ of the primitive hypoblast, and that the cells of this 
layer assume a somewhat more flattened form at the peri- 
VOL, XXXII, PART 111.—NEW SER. BEE 
