THE FCETAL MEMBRANES OP REPTILES AND EIEDS. 



213 



yolk becomes overgrown and as the vascular area extends farther 

 downwards. Parietal and visceral middle layers separate more and 

 more from each other, until finally (in the Chick toward the end of 

 incubation) a separation results over the entire periphery of the yolk- 

 sphere. Figs. 3, 4, and 5, Plate I., show stages in this process. In 

 the last figure, which represents the condition on about the seventh 

 day of incubation, the extra-embryonic part of the body-cavity has 

 already become very considerable ; the serous envelope is, with the 

 exception of a small place at the vegetative pole of the yolk, every- 

 where formed as a separate structure. 



In connection with this the wall of the yolk-sac also becomes 

 changed. Whereas at the beginning of the overgrowth it embraces 

 for a considerable distance all the germ-layers, after the separation 

 of the serosa it is composed exclusively of entoderm and the visceral 

 middle layer. 



Explanation of the Figures on Plate I. 



Figs. 1-5 are diagrammatic representations of cross and longitudinal sections 

 throiigli the Hen's egg at different stages of incubation. They are intended to 

 illustrate how the body of the Chick is developed Out of the embryonic funda- 

 ment, and how the yolk-.'-ac, the amnion, the serosa, and the allantois arise out 

 of the extra-embryonic area of the gefin-layers. 



For the sake of clearness the embryonic fundament, aind later the embryo, 

 are represented much too large in relation to the yolk. 



In order more easily to distinguish the different parts from one another 

 different colors have been selected for them. The yolk Is represented in 

 yellow, the entoderm green, the outer germ-layer blue, and the middle germ- 

 layer, together with the mesenchyme, red. The black dots indicate the limit 

 to which the outer and inner germ-layers have grown over the yolk in the 

 different stages ; the red dots mark the boundary for the time being of the 

 middle germ-layer, which after the development of the blood-vessels ends in 

 the sinus terminalis. 



The references a/pply 



akj Outer gerrti-layer (blue). 



i»w, Medullary lidges or folds. 



N, Neural tube. 



«/, Amniotic fold. 



i?ff/, Aa/, «fl/, Anterior, posterior, and lateral 



amniotic folds. 

 A, AmiUoD. 

 all, Amniotic cavity. 

 S, Serous membrane (Serosa), 

 /m. Dermal umbilicus. 

 sf, Lateral £(9ld8. J/', A/=, Head-fold ; a/J, i/d, 



outer and inner limbs of fold, 

 it, Inner geiin-layer (green). 

 «r. Its margin of overgrowth. 

 dr. Intestinal groove. 



to all of the figures. 



dg, Vitelline duct. 



al, Allantois. 



ih, Intestinal sac. 



dn, Intestmal umbilicus. 



'ink, MMdle germ -layer (red). 



mh S Parietal lamella of .the same or parietal 

 middle layer, 



mi^, Visceral lamella of the siime or visceral 

 middle layer. 



St, Lateral limit of the same, sinus terminalis, 

 marginal vein. 



dm, vm. Dorsal and ventral mesentevies. 



Ih, Body-cavity. W, Embryonic, Ih-, extra- 

 embryonic part of the same. 



