62O 



REPTILIA. 



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 pellu- 

 cida. 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. 

 It is here that the fcetal membranes 

 known as amnion and allantois are first 

 formed. 



(a) The Amnion. At an early stage 

 in development the head end of the 

 embryo seems to sink into the subjacent 

 yolk. A semilunar fold of the blastoderm, 

 including epiblast and mesoblast, rises 

 up in front. Similar folds appear later- 

 ally. All the folds increase in size, arch 

 upwards, and unite above, forming a 

 dome over the embryo. Each of these 

 folds is double ; the inner limbs unite to 

 form " the true amnion " ; the outer limbs 

 unite to form "the false amnion," 

 "serous membrane," or subzonal mem- 

 brane. The cavity bounded by the true 

 amnion contains an amniotic fluid bathing 

 the outer surface of the embryo ; the 

 cavity between the true and the false 

 amnion is lined by mesoblast, and is 

 continuous with the pleuro-peritoneal or 



FIG. 306. Origin of amnion and 

 allantois. After Balfour. 



Rise of amniotic folds (a.f.) around embryo 

 (*') //> pleuroperitonealjCavity ; y., yolk. 



Further growth of amniotic folds (ji.f.) over 

 embyro and around yolk. 



Fusion of amniotic folds above embyro ; 

 ^./., amnion proper ; s.z.jn., subzonal mem- 

 brane ; j'.s. , yolk-sac. 



Outgrowth of allantois (/.) ; amniotic cavity 

 (a.c.) ; //., head end ; /., tail end. 



Complete enclosure and reduction of yolk-sac 

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

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

 embryo. 



