414 General Notes. [ April, 
fish embryo. It is therefore interesting to note that an actual con- 
crescence from behind forward of this rand-wulst or lower layer 
or a proliferation of cells from behind forwards would not be im- 
possible. The zxner mass of cells of the Mammalian ovum while 
in the vesicular or blastodermic stage is evidently in part homol- 
ogous with some part of the thickened rim of the blastoderm of 
lower forms. 
It is also a matter of great interest in this connection to observe 
that in the Sauropsida the rand-wulst or germinal wall is not 
carried along with the extreme edge of the epiblastic stratum 
quite to the border of the blastoderm all round as in Ichthyop- 
sida. The epiblastic layer of the blastoderm in the Sauropsida 
rapidly extends beyond the lower layer or germinal wall, leaving 
it more or less remote from the outer margin of the germinal 
area. This peculiarity of development alone would be sufficient 
to cause the embryo to be formed away from the margin of 
the blastoderm in the Sauropsida, but even this I venture to 
Suggest is to be explained by the increase in the size of the yolk 
of the ova of Sauropsida, the connecting link between the latter, 
and the teleporous Teleostean ovum being supplied by that of 
the Elasmobranchs, which probably represents at least one of the 
steps by which the evolution of the blastoderm of Sauropsida and 
Mammalia was attained, although it would obviously be incor- 
rect to assume that these stages of blastodermic evolution were 
indicative of a serial or successional affiliation through descent. 
It would probably be much more rational to regard the develop- 
ment of these differences as being in the main due to an increase 
in the volume of the yolk as urged by Balfour, and that the 
causes of variations in its development were therefore to some ex- 
not the true blastopore, and if it can be regarded as representing 
the yolk blastopore, which seems very probable, the zzner mass of 
cells finally involuted on its closure or covered over by the epi- 
bolic growth of the epiblast, and from which mass the mesoblast 
and hypoblast are derived, that mass becomes homologous with 
the marginal lower layer or rand-wulst of such a type as the 
_ Teleostean ovum. 3 
‘The degeneracy of the vitellus of the ovum of Mammalia may 
_ Possibly be due to the development of the so-called uterine milk 
from the uterine glands by which the egg is nourished from with- 
_ out during a very early stage and before the development of the 
area vasculosa or the vessels of the allantois is accomplished. 
