642 



REPTILIA. 



Development of Reptiles 



The ovum contains much y.olk, 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 blastopore 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. 



FlG. 353. Origin of amnion and 

 allantois. After Balfour. 



i. Rise of amniotic folds (a.f.) around embryo (e) ; 



/./., pleuro-peritoneal cavity ; jj'., yolk. 

 9. Further growth of amniotic folds (a-f.} over 



embryo and around yolk. 

 3. Fusion of amniotic folds above embryo ; a.p., 



amnion proper ; s.z.m., subzonal membrane ; 



z.m 



y.s., yolk-sac. 

 4. Outj 



utgrowth of allantois (#/.) ; amniotic cavity 

 (a.c.) ; h., head end ; t., tail end. 

 5. Complete enclosure and reduction of yolk-sac 

 (y.s.); s.z.m., subzonal membrane; a.p.^ 

 amnion proper ; #/., allantois ; g. , gut of 

 embryo. 



