590 



REPTILES. 



simply forms a sac round the yolk. One of the first signs of develop- 

 ment 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 

 hrst 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, con- 

 stricting it off from the subjacent yolk 

 or yolk sac. 



Fatal Membranes. — It is \\'ith Rep- 

 tiles that the series of higher Verte- 

 brates or Amniota begins. It is here 

 that the fcetal membranes known as 

 amnion and allantois are first formed. 

 Let us consider their development. 



(a) The Amnion. — At an early stage 

 in development, the head end of the 

 embryo seems to sink into the sulijacent 

 yolk. A semilunar fold of the blasto- 

 derm, including epiblast and mesoblast, 

 rises up in front. Similar folds appear 

 laterally. 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 sub- 

 zonal membrane. 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 meso- 

 blast, and is continuous with the pleuro- 



FlG. 204. — Origin of Amnion and 

 Allantois. (After Balfour.) 



1. Rise of amniotic folds (a.y;) ; .-iround em- 

 bryo (f) ; /./., pleuro-peritoneal cavity; y., 

 yolk. 



2. Further growth of amniotic folds (<z.A) 

 over embryo and around yolk. 



3. Fusion of amniotic folds above embryo 

 <!./., amnion proper; .v.c.w., sub-zonal mem- 

 brane ; _j'..v., 5'olk sac. 



4- Outgrowth of allantois (ij/.) ; amniotic 

 cavity (^.c.) ; A., head end ; t., tail end. 



5. Complete enclosure and reduction of yolk 

 sac {_i'.j-.) ; s.z.jn.^ sub-zonal membrane; a./., 

 amnion proper ; a/., allantois : ^., gut of 

 embryo. 



Sim 



