REPRODUCTION AND LIFE CYCLES 



125 



The first of these new membranes to be considered are the amnion 

 and chorion. They may be best understood by studying their origin. 

 It will be recalled that in telolecithal eggs the endoderm does not 

 succeed at once in growing ventrally to meet, and so to close, the 

 digestive tube. Instead the unclosed tube lies flat upon the surface 

 of the yolk. Both the ectoderm and mesoderm grow laterally over 

 the endoderm dii'ectly over the yolk on the inner layer of the blasto- 

 derm. The mesoderm as a whole divides into three portions, the 



tmbTYo.. 



ectoderm 



.Tnesoderni 



...endbcterm. 



coelom. 



allantoic cavity 

 amnion \ aWantois 



amniotic- 

 cavitv 



znhryoy 



shell 



niembn 



olbixmen...'? 



embryo. 



■c.^voll-c" 



chorion! - 



extra--' 



embr/onid 



coelom 



">.,viLe!Une 

 mambrana 



,ai-nniotic Cavity 



allantois 



ommotic 

 CCLVity.. 



choriotv 



allantpis 

 chorion., 

 aranion 



yolk 

 vitelline 

 menabrans 



.^ allantoic cavity^^^^.^ sWlc 

 ' ,">tolk scut. 



--yolk 



^/itilli-ne 

 iTjsmbran 



._yolk' 



vitelline 

 iriembrane 



Development of the extra-embryonic membranes in the chick. State the 

 contribution of each germ layer to the amnion, chorion, and yolk sac. (After 

 Patten.) 



first of which is the upper epimere part immediately flanking the 

 developing neural tube and producing the somites. Beneath the epi- 

 mere lies a small mesomeral portion that later develops the excre- 

 tory and reproductive systems from a ridge lying in the dorsal wall of 

 the coelom. The mesoderm below the mesomere is the hypornere, 

 which soon divides into an outer somatic and inner splanchnic layer 

 of mesoderm. In large-yolked eggs this hypomeral portion extends 

 laterally over the endoderm which is covering the surface of the 

 yolk. In all of the higher groups, beginning with the reptiles, the 



