214 EMBRYOLOGY. 



Fig. 1. Cross section through a Hen's egg on the second day of incubation. 



The germ-layers are spread out flat over the yolk ; the middle one is less 

 extensive than the other two. The first blood-vessels have developed, and 

 terminate with the marginal vein (st) at the edge of the middle germ-layer. 

 One now distinguishes therefore the vascular area, which extends to the red 

 dotted line (st), and external to it the yolk-area (dh), which terminates with 

 the black dotted line (ur), the edge of overgrowth of the outer and inner 

 germ-layers. 



Fig. 2. Cross section through a Hen's egg on the third day of incubation. 



The outer and inner germ-layers are spread out over half of the yolk. 

 The yolk -area (dh) terminates with the black dotted line (ur), the edge of 

 overgrowth. 



The middle germ-layer, with the vascular area, which is now well developed, 

 has also grown over the yolk as far as the line st (the sinus terrninalis). In 

 the middle germ-layer the body-cavity has become distinct in the embryonic 

 region (Ik 1 ) and in its immediate vicinity (77i 2 ), the parietal (mk l ) and visceral 

 middle layers (mW) having separated from each other. 



The embryonic fundament begins to be constricted off from the extra- 

 embryonic part by a process of folding and to constitute the trunk. The lateral 

 folds (sf) have grown downwards for a certain distance, thus giving rise to 

 the lateral walls of the trunk, whereas ventrally the body is still open. Corre- 

 sponding to these lateral folds (sf), the lateral intestinal folds (df) have 

 arisen on the splanchnopleure, and bound the intestinal groove (dr). 



The embryo in process of being constricted off has sunk into a depression of 

 the more and more liquefied yolk, and becomes partly enveloped by the somato- 

 pleure of the extra-embryonic area of the germ-layers, the lateral folds of 

 the amnion (of) having already encircled the sides of the embryonic body. 



Fig. 3 shows a longitudinal section through the stage represented in cross 

 section in fig. 2. (Third day of incubation.) 



The head-end of the body is entirely constricted off from the blastoderm. 

 It encloses the cephalic portion of the intestine (Kopfdarmhb'hle). The tail- 

 end is only slightly differentiated. The anterior fold of the amnion (vaf) has 

 invested the head, the posterior fold (haf) the tail (cephalic sheath, caudal 

 sheath). 



The middle of the trunk is still wide open ventrally. The place where 

 the body-wall passes over into the folds of the amnion, and which is indicated 

 in the diagram by the ring hn, is called the dermal umbilicus. 



The splanchnopleure has become closed into a tube anteriorly and pos- 

 teriorly (the cephalic and pelvic portions of the intestinal cavity) ; in the 

 middle the tube is still open ventrally, and by means of the vitelline duct (dg) 

 is continuous with the yolk-sac (ds). The place of transition indicated by 

 the ring dn is the intestinal umbilicus. The allantois (al) grows out as a small 

 vesicle from the ventral wall of the pelvic portion of the intestinal cavity into 

 the body-cavity of the embryo. 



