THE HUMAN YOLK-SAC. 



355 



stained in ioto with alum cochineal, imbedded in paraffin, and cut in transverse 

 sections. Yolk-sacs of the second month are preferable for study. 



The general history of the yolk-sac is described on pages 85 and 89. 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 1 1 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 

 disappearing. In this condition we found the yolk-stalk in an embryo of 2 1 mm. 

 (Fig. 51, A). The sac itself remains hollow (Fig. 216). It has a lining of ento- 

 dermal cells, En, and a thicker layer of meso- 

 derm, mes, containing blood-vessels, d. The 

 network of the vessels imparts a characteristic 

 appearance to the external or mesodermic sur- 

 face of the yolk-sac. In the earliest stages ob- 

 served the entoderm consisted of a single layer 

 of cuboidal cells. 



TransTjerse 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 entoderm has undergone prolif- 

 eration and thickening. 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-corpuscles. 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. 216. — Section of the Yolk-sac 



OF A Very Young Human Embryo. 



£n, Entoderm. mes, Mesoderm. v, 



Blood-vessel.— (4r/'<f/- Fr. Keibel.) 



