Tin- in I'M. MI-:MI:I: UHK 01 UQ -1 '* 



yolk In conn > ov. Thrown and as the vascular area i-\trinU farther 

 downuanK. I'ai-i'Mal a in I \ !>< ra 1 middle l;i\ i :ite more a lid 



in<re fivin i ach other, until 1'mally (in tin- Chirk toward the end of 

 incubation) a SI-JIM ration n suits over the entire periphery of the yolk- 

 sphere. Kiirs. '.'i, -1, and .". Plat.- I.. .-how gtaget in this pfO06tt. In 

 tin- last figure, which repn s.-nts the condition on ahont t he >evenf h 

 3my of incubation, the extra -embryonic part of the body ca\ ity has 

 already become very considerable ; the M-PMIS envelope i-, with the 

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

 where formed as a separate structure. 



In connection with this the \\all of the yolk-sac also becomes 

 changed. Win reas at the beginning of the overgrowth it embraces 

 for a considerable distance all the p-rin -lavers. after the srjiai 

 of the serosa it is composed exclusively of entoderin and the vi 

 middle lav-r. 



EXPLANATION OF THE FIGURES ON PLATK I. 



Figs. 1-." arc diagrammatic representations of cross and longitudinal sections 

 through the Hen's egg at different stages of incubation. They are intm 

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

 ment, and how the yolk-sac, the amnion, the serosa, and the allantois arise out 

 of tin- extra-embryonic area of the germ-la;. 



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 

 cnt 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 terminals. 



The references apphj to all ofthetl,; 



<tk, Outer germ-layer (blue). i dp, Vit.-lliiu- duct. 



' In ll.-iry ridges or folds. /, Al];ui' 

 .V. N. ur.il tube. r.'-tinnl sac. 



miotio t"..lil. 

 vuf, haf, tof, Anterior, posterior, and lateral ink-, M < >ter (red). 



ninniotiu folds. i it" the same 01 



A, Amni..n. inul.ll.- 1.^ 



S, Serous membrane (Seroea). 



iinal umbilicus. 

 tf, Lateral folds. */', tf, H.-ad-f.-M : nib, ifb, 



outer and inner limbs of fold. 

 ik, Inner germ-layer (green). 

 vr, Its margin of overgrowth, 

 tfr, Intestinal groove. 



mk*, Visceral lamella of the aan 



luul.llc 1 i 

 It, Lateral limit of the same, sinus t-i i 



nmrginal vein. 



Dorsal and ventia! 

 Ik, Body-cavity. lh\ Embryonic, Ik*, extra- 



enibry..iii.- pait of tin- .-ame. 



