A HUMAN EMBRYO OF THE PRESOMITE PERIOD. 407 



Owing to the uniform distribution of these different types of vessels throughout 

 the chorion, there is strong evidence of the general differentiation of blood-vessels 

 in loco rather than from a single focus restricted to any particular area. 



In the region of the body-stalk one can recognize two areas of mesoderm: a 

 more condensed area immediately surrounding the allantoic stalk, whose lateral 

 margins are definitely inclosed by the formation of mesothelium and a much less 

 compact area intervening between the former and the chorionic membrane. The 

 more compact area is doubtless to be regarded as the forerunner of the permanent 

 umbilical cord, whereas the looser area eventually is taken up by the exocoelom. 

 In both of these regions blood-vessel formation can be seen taking place, and is 

 in about the same degree of development as that noted in the chorionic membrane 

 and villi. In some places throughout the looser areas of the body-stalk are small 

 endothelial vesicles, which are completely detached from the adjoining exoccelom. 



Blood-vessel formation can be detected over the greater part of the parietal 

 mesoblast covering the amnion. In the body-stalk, as in the chorion, young blood- 

 vessels are for the most part empty. 



Angiogenesis in the yolk-sac presents a somewhat different picture from that 

 seen in other parts of the ovum. In the first place it is circumscribed, being limited 

 to the caudo-ventral half of the yolk-sac and is most marked at the extreme caudo- 

 ventral pole; in the second place, the angiogenetic picture is quite different from 

 that described as typical for the chorionic villi. 



The wall of the yolk-sac consists of a thin, stretched-out, endodermic mem- 

 brane, which is shown in figure 23, plate 5. This is covered in by the visceral 

 mesoblast, shown below in the figure. In the more dorsal part of the yolk-sac the 

 mesoblast is much thicker and more membranous and is separated from the endo- 

 derm by a distinct, narrow cleft. The cleavage between these two extends two- 

 thirds of the distance from the dorsal pole to the equator and entirely encircles the 

 embryonic area. Ventral to the cleavage rim a thin coating of visceral mesoblast 

 fuses tightly with the endoderm, in some places being so thin that one can scarcely 

 be sure that there is anything more than endoderm present. The simplicity of these 

 regions of the wall of the yolk-sac constitutes very favorable conditions for the 

 study of blood-vessel formation. 



Sketches were made of selected areas of the wall, showing blood-vessels in their 

 different stages. These are arranged in figures 23 to 28, plate 5, in their approximate 

 order of development. All of the figures are so arranged that the endoderm is above 

 and the mesoderm below. In figure 23 is shown a small, isolated clump of pro- 

 liferating mesoblast cells in an area where the endoderm is only scantily covered. 

 From such an angioblastic node we can find all stages of transition up to completed 

 endothelial tubes. In figure 24 is another clump, slightly larger, but otherwise of 

 much the same character. From their appearance one could not be sure that such 

 clumps were destined to form blood-vessels. Since, however, there is at this time 

 no other process taking place, we may assume that these are earlier stages of the 

 condition met with in figure 25. In this figure there is shown a characteristic, com- 

 pact, multinuclear plate, in which the cellular boundaries can be only partially 



