A HUMAN EMBRYO OF THE PRESOMITE PERIOD. 405 



ANGIOGENESIS. 



Evidences of blood-vessel formation can be recognized in all parts of the 

 chorion and the body-stalk and in certain areas of the yolk-sac. The mesoderm of 

 the latter is very different in character from that of the body-stalk and chorion. 

 To a lesser degree the mesoderm of the body-stalk and chorionic membrane differs 

 from the mesodermic stroma of the villi; thus the picture of developing blood- 

 vessels varies according to the region examined. We will therefore consider first 

 the conditions existing in the chorion and body-stalk, which are closely allied, and 

 then proceed to the vessel-formation of the yolk-sac, which has its own peculiar 

 type of development. 



As regards the chorionic membrane and villi, a careful survey shows very little 

 difference in the number or degree of development of the blood-vessels in the 

 different regions. In all parts of the chorion one can find the earliest types of 

 vessels, consisting of simple protoplasmic, multinucleated strands, and the inter- 

 vening stages between this and completed endothelial tubes. Selected stages of 

 this process were sketched and are shown on plate 5, figures 17 to 22, which are 

 arranged according to their apparent degree of differentiation. One gains the 

 impression that the vessels are more numerous in the villi than in the chorionic 

 membrane. This may, however, be due to the fact that the chorionic membrane 

 is compressed into a compact sheet, consisting of 3 to 5 layers of flattened nuclei 

 and their intermediate process-like strands. Owing to their arrangement these 

 nuclei resemble endothelium, which makes the identification of endothelial forma- 

 tion uncertain. At the points where the mesoderm of the chorionic membrane 

 evaginates to form the stroma of the villi, the trabeculse of the tissue are somewhat 

 more loosely arranged and this makes it possible to identify the younger vessels 

 with more certainty. The villi themselves offer a most favorable place for the 

 study of angiogenesis. The arrangements here are extremely simple and the 

 topography of the villi is such that one can select either cross or longitudinal sec- 

 tions at will. We have to take into consideration only a rather uniform stroma of 

 the villus and its double-layered covering of epithelium. In the meshes of the 

 stroma can be found the condensed strands which represent blood-vessels in their 

 various stages of formation. 



In the specimen we are describing, a great many of the villi do not show any 

 sign as yet of blood-vessels. The villi devoid of blood-vessels are, as a rule, the 

 smaller ones, and these are distributed evenly over all parts of the chorion. In 

 some cases one can find a section showing a large villus with a mature main vessel, 

 from which less mature strands can be seen extending into the terminal branches 

 of the villus. These strands may be in the center of the villus or may extend 

 obliquely so as to terminate close against the epithelium. The process by which 

 blood-vessels are formed in the mesodermal tissue of the villi is initiated by the 

 condensation of slender, cytoplasmic strands, along which are scattered irregularly 

 placed and actively proliferating nuclei. We may speak of these as angioblastic 

 strands which can be only incompletely resolved into separate cells. The next 

 stage in the process consists in the differentiation of some of the component parts of 



