376 



HUMAN UTERUS AND FETAL APPENDAGES. 



The general history of the yolk-sac is described on pages 63 and 66. It 

 becomes a pear-shaped vesicle which in man attains its maximum diameter about 

 the end of the fourth week. It then measures from 7 to n mm. From its 

 pointed end runs the long stalk by which it is connected with the intestine. In 

 very early stages the stalk is hollow and its cavity is lined by entoderm. But 

 this condition is soon obliterated, the stalk becoming solid and the entoderm dis- 

 appearing. In this condition we found the yolk-stalk in an embryo of 21 mm. 



(Fig. 66, A). The sac itself remains hollow (Fig. 

 260). It has a lining of entodermal cells, En, and 

 a thicker layer of mesoderm, mes, containing blood- 

 vessels, v. The network of the vessels imparts a 

 characteristic appearance to the external or meso- 

 dermic surface of the yolk-sac. In the earliest 

 stages observed the entoderm consisted of a single 

 layer of cuboidal cells. 



Transverse Section of a Yolk-sac of about Two 

 Months. The contents of the fresh yolk-sac are 

 fluid, but coagulate when the organ is hardened. 

 In the coagulum are found some stained bodies 



_ which are supposed to be yolk material. The 



OF A VERY YOUNG HUMAN EMBRYO. J 



En, Entoderm. mes, Mesoderm. v, entoderm has undergone proliferation and thick- 

 Blood-vessel. (After Fr. Keibel.) ening. These cells are more or less irregular and 



disposed in two or three layers. Many of the 



superficial cells are stained deeply and have small nuclei, while the deeper lying 

 cells are larger, more lightly stained, and have larger nuclei and more distinct cell 

 boundaries. The mesoderm consists chiefly of somewhat crowded mesenchymal cells, 

 the nuclei of which are smaller than the entodermal cells, and a well-marked layer of 

 mesothelium, which forms the external covering of the yolk-sac. In the mesoderm 

 appear relatively large blood-vessels, which are usually found filled with blood-cor- 

 puscles. The blood-vessels have distinct endothelial walls and lie in the part of 

 the mesoderm toward the. mesothelium, so that they are separated somewhat from 

 the entoderm and seem often to lie immediately underneath the mesothelium. They 

 are so large that each vessel causes a protuberance upon the yolk-sac. 



FIG. 260. SECTION OF THE YOLK-SAC 



