642 REPTILIA. 
Development of Reptiles 
The ovum contains much yolk, at one pole of which there is a small 
quantity of formative protoplasm surrounding the germinal vesicle. 
The segmentation is necessarily meroblastic and discoidal, as in 
Birds. 
The segmented area or blastoderm, originally at one pole, gradually 
grows round the yolk. The central region of the dorsal blastoderm 
is separated from the yolk by a 
shallow space filled with fluid, and 
is clearer than the rest of the blas- 
toderm. In this central region or 
area pellucida, the germinal layers and 
subsequently the parts of the embryo 
are established, while the rest of the 
blastoderm — the area opaca — simply 
forms a sac round the yolk. One of 
the first signs of development is the 
appearance of a thickened band of cells 
extending forward in the middle line from 
the posterior margin of the area pellucida. 
This band is called the primitive streak, 
and seems to represent a fusion of the two 
edges of the blastoderm behind the future 
embryonic region. The embryo develops 
in front of the primitive streak, and one 
of the first signs of its development is the 
formation of a primitive or medullary 
groove in a line with the primitive streak. 
As development proceeds, folds appear 
around the embryo, constricting it off 
from the subjacent yolk or yolk-sac. 
It is with Reptiles that the series of 
higher Vertebrates or Amniota begins. 
Fic. 353.—Origin of amnion and 
allantois. —After Balfour, 
1. Rise of amniotic folds (@.£) around embryo (¢) ; 
pp.) pleuro-peritoneal cavity ; 4, yolk. 
z. Further growth of amniotic folds (a) over 
enibryo and around yolk. 
3. Fusion of amniotic folds above embryo ; a.f., 
amnion proper ; s.z.7#., subzonal membrane ; 
ys., yolk-sac. 
4. Outgrowth of allantois (a/.); amniotic cavity 
(a.c.); #., head end; z., tail end. 
5. Complete enclosure and reduction of -yolk-sac 
(y.s.)5 $.2.22., subzonal membrane; a.J., 
amnion proper; aé., allantois; &, gut of 
embryo. 
