DEVELOPMENT OF GERMINAL LAYERS IN MAMMALS. 417 
In the former, however, the earlier stages have probably been 
shortened, for no one appears to have observed the closure of 
the blastoporic lips; but in birds Duval (10) has described the 
process of concrescence which precedes the formation of the 
primitive streak. 
The tendency to early separation of epiblast and hypoblast 
in the region of the blastopore has been transmitted from the 
Protamniota, not only to the Lacertilia, but also to Mammalia ; 
but in the latter order, due probably to the altered condition 
in which the ovum is placed during the early stages of its 
development, many other modifications of the ordinary de- 
velopmental processes also appear. Dealing, however, with 
the rat, mouse, and hedgehog only, it is not difficult to trace a 
fairly close correspondence between mammalian and lacertilian 
development. If an ovum with a transmitted tendency to 
comparatively great hypoblastic proliferation becomes em- 
bedded, at an early period, in a decidual tissue from which 
nutriment can be easily absorbed, it is quite possible that the 
hypoblast may acquire an attachment to the uterine tissues, 
in which case the spread of the epiblast will be prevented and 
the yolk blastopore will remain permanently open, asin the rat 
and the mouse. Under these circumstances solid yolk forma- 
tion will no longer be necessary, and it will probably cease ; but, 
as Hubrecht has already pointed out (23, p. 529), the exposure 
of a large ovular surface to the maternal tissues would still be 
a distinct advantage; and thus, instead of the yolk-sac disappear- 
ing in the passage from the Protamniota to the Mammalia, it 
has been retained, and has acquired the form of a large hollow 
sac, which becomes filled and distended by maternal fluids passed 
into it either by secretion or osmosis. Nevertheless even in 
the rat and the mouse the yolk-sac is relatively small when 
compared with the yolk-sac of the Lacertilia, and there can be 
but little doubt that during the passage from the meroblastic 
Protamniota to the holoblastic mammal a considerable reduc- 
tion in the rapidity of hypoblastic proliferation has occurred ; 
and this reduction has been so great in the hedgehog that at 
an early period the yolk blastopore becomes closed by the 
