THE FCETAL MEMBBANES OF REPTILES AND BIRDS. 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 

 through 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-sac, the amnibn, the serosa, and the allantois arise out 

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



For the sake of clearness the embryonic fundament, and 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 apply to all of the figures. 



ale, Outer germ-layer (blue). 

 mw, Medullary ridges or folds. 

 N, Neural tube, 

 a/, Amniotic fold. 



dg, Vitelline duct. 

 al, AUantois. 

 ds, Intestinal sac. 

 dn, Intestinal umbilicus. 



va/, liaf, saf, Anterior, posterior, and lateral j mk, Middle germ-layer (red). 



araniotic folds. mk ', Parietal lamella of the same or parietal 



A, Amnion. 



<th, Amniotic cavity. 



S, Serous membrane (Serosa). 



Jin, Dermal umbilicus. 



tf, Lateral folds. lrf l , kf, Head-fold ; a/6, if It, 



outer and inner limbs of fold. 

 ik, Inner fferm-lctyer (green), 

 wr, Its margin of overgrowth. 

 <lr, Intestinal groove. 



middle layer. 



mk', Visceral lamella of the same or visceral 

 middle layer. 



st, Lateral limit of the same, sinus terminal!*, 

 marginal vein. 



dm, vm, Dorsal and ventral mesenteries. 



Ih, Body -cavity. lh\ Embryonic, IK', extra- 

 embryonic part of the game. 



