mi: rOBTAI MI:MI;I:\M D i;illD8. iM 1 



and JJiids. it i> to be 8660 t hat a eon.Mderal.Io complic ii ion ha- ft] 

 in the case of tlie latter. Whereas in Ki-ln-s die extra-em 1>;\.. nil- 

 area of tin- M.matopleure becomes excln>i\. 1\ tin- deimal yolk-M 

 Keptiles and l!ird> t \\ o sacs have arisen out of it l.v a process of 

 folding. The influences producing this folding be clear. 



Since the egg is enclosed in liriuly applied rim -lopes, the embryonic 

 liody. when it i> formal by tin- folding together of the germ 1: 

 cannot rise from the yolk-sac ; it therefore comes to lie in a dcj 

 MOII of the latter. There is the more reason for tin- e of 



this because the embryo at the beginning of development i- 

 Mvely Miiall in comparison with the yolk, and herau>e the yolk 1, 

 immediately underlying it become liquefied and absorbed. With 

 the sinking of the body into the yolk (Plate I., figs. 2 and 3), the 

 parts which in Fishes become the simple dermal yolk o I., 



'' and 7) fold in around it on all sides as amniotic fold-, and 

 enclo.M- it the more completely the deeper it >ink> into the yolk 



The preceding account of the development of the araniou is u. 

 \\luit schematic in a Miiirle pnint. 'J'hat is to say, the antrrior fold of the 

 aniniun is developed so early, that the middle germ-lay. : yet been 



ai>l> to spread out as far as the anterior part of the embi : The in- 



folding, therefore, in this region involves only the outer and inner germ-layers, 

 \\hic':i are still rlo-dy united. This condition h changed sonif.viiat 

 when the middle germ-layer has grown into the region of the anterior fold of 

 t-lie amnion, anl has there split into a visceral and a parietal layer. 1 

 has not yet been followed out in detail in series of longitudina 15ut 



e\ent-j we must assume that the entoblast, which is m.ittil with the 

 -al mi<l<lle layei-. retracts from the anterior fold of the amnion ami 

 spreads out ilat, as is represented in diagrammatic figure 11 (Plata Ij. In 

 this manner the anterior amniotic fold, which in the meantime lias become 

 I, now consists of the outer germ-layer and the parietal middle 

 layer. a> i> the case from the beginning with the subsequently arising posterior 

 and lateral fold> of the amnion. 



\\ no\\- have to ent<-r still more particularly upon the further 

 relations of amnion and serO8&. 



Up to the end of embryonic development th ttmij Bmainfl 



ill continuity with a small region on the ventral >ide of the ein' 

 which i> called the dermal umbilicu>. In figs. 3, 4, 5, and 10 

 (Hate I.) this plaee is indicated by mean- of a aroolai '. 

 Here the piimitive la\er> of the body-wall are continuous with the 

 Corresponding layers of the amnion. as. for instance, the epidermis of 

 the body with an epithelial layer lining the amniotic cavity, 

 dermal umbilicus of Reptiles and Uirds corresponds therefore with 



