420 ARTHUR ROBINSON. 
round the inner surface of the epiblast. The figures which 
are given as representations of the phenomenon do not afford 
conclusive proof of its occurrence ; in point of fact, they are as 
adaptable to an opposed hypothesis as to that which they are 
commonly supposed to support. 
Since the publication, in 1880, of van Beneden’s mono- 
graph on the early development of the rabbit’s ovum (2), the 
epiblastic nature of the greater part of the wall of the primi- 
tive blastocyst has been almost universally accepted; most of 
the accounts of mammalian development which have been 
published since its appearance have been correspondingly 
modified. 
According to van Beneden’s description the first division 
separates the ovum into two segments of unequal size and of 
different appearance. The descendants of one of these seg- 
ments at the seventy-second hour form a mass of cells which 
are completely surrounded by the descendants of the other 
segment. Van Beneden termed the outer layer the epiblast 
and the inuer mass the primitive hypoblast, and his figures 
(figs. 1 and 4, Pl. IV) represent these two constituent por- 
tions very clearly. At the period of the publication of his 
memoir van Beneden believed that the inner mass eventually 
gave rise to the definite hypoblast and the mesoblast, and that 
the outer layer formed the epiblast. It has, however, been 
conclusively proved, by the observations of Kolliker, that from 
the inner mass all three layers of the germ arise, and it is 
doubtful whether the outer layer takes part in the formation 
of the germinal epiblast (17) or whether it disappears entirely 
in the germinal area (27, p. 33). However this may be, the 
evident difficulty which exists in the determination of the 
question is an indication that differences of appearance 
cannot alone be taken as sufficient evidence of the special 
nature of the cells under observation ; yet it is mainly upon 
such differences, seen in optical sections only, that van Be- 
neden bases his description of an outer epiblastic layer and 
an inner hypoblastic mass in the very early stages. Further, 
these differences do not seem to have been noticed by Rauber, 
