490 REPTILES. 



The ova contain much yolk, at one pole of which there is a small 

 quantity of formative protoplasm surrounding the germinal vesicle. 

 Formation of polar globules has not been observed. 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 blastoderm. 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 around 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 

 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. 



Festal Membranes. — It is with Reptiles 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 develop- 

 ment. 



[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 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 mesoblast, and is really an extension of the body-cavity of the 

 embryo. The amniotic folds extend not only over the embryo, but 

 ventrally around the yolk-sac which they completely invest. 



{b) The Allantois. — While the. amnion is being formed, a sac grows 

 out from the hind end of the embryonic gut. This is the allantois, lined 

 internally by hypoblast, externally by mesoblast. It rapidly insinuates 

 itself between the two limbs of the amnion, eventually surrounding both 

 embryo and yolk-sac. 



The amnion is a protective membrane, forming a kind of water-bag 

 around the embryo. It seems to be due in part to the embryo sinking 

 into the yolk-sac by its own weight. 



The allantoic sac is vascular, and has respiratory and perhaps also 

 some yolk-absorbing functions. It seems to be homologous with the 

 outgrowth which forms the cloacal bladder of Amphibians ; it is often 

 called " a precociously developed urinary bladder." 



Before the amnion is developed, the heavy head end of the embryo, 

 has already sunk into a depression (in Lizards, Chelonians, Birds (?) 



