86 A HUMAN EMBRYO AT THE BEGINNING OF SEGMENTATION, 



other cell-layers of the embryo except the medullary folds and otic plates. Closer 

 examination of this surface, however, reveals a fine cremation of the edge on section, 

 representing the points of union between the fibrils and the mesothelial cells of 

 which they constitute a part (cf. Szily, 1. c, 1908, fig. 6). Anteriorly, where the 

 fibrillar network loses its delicate, uniform characters, the under surface of the 

 mantle suffers in a similar way, becoming very irregular and uneven. The varia- 

 tions in the nature of the mesostroma in different regions of the embryo are essen- 

 tially quantitative in character. All conceivable gradations are represented, as 

 may be seen in the examples furnished by Szily. Free cells or small cell-masses in 

 the reticulum are practically restricted to the anterior third of the plexus, and here 

 again they are more in evidence around the caudal limits of the plexus, where con- 

 siderable remodeling will take place, than farther forward where definite blood- 

 vessels represent the beginning of the first arches. 



The free, ccelomic surface of the mantle is relatively rough and uneven ; the 

 cells are frequently clumped together in small (at times hollow) masses projecting 

 into the pericardial cavity. The mantle is thickest behind and gradually thins out 

 to the line of reflection as the parietal layer at the top of the pericardial cavity. 

 Details of its structure can not be made out; in its anterior portion there is but a 

 single layer of cells; elsewhere and particularly low down there may be more than 

 one. Nowhere, however, is there any break in the clear-cut basal surface of the 

 mantle or any indication on the part of its cells of an invasion of the underlying 

 reticulum. The parietal layer of the pericardial mesothelium is a single layer 

 throughout. Thinnest ventrally, it becomes thicker over the lateral walls, while 

 dorsally and where it forms the mesocardium its cells are cuboidal or low columnar 

 in type. In the dorsal recesses, where they are expanded to join the general pericar- 

 dial cavity, their walls exhibit considerable irregularity in the form of deep, nar- 

 row clefts or tiny outpocketings lined by low columnar cells. 



A consideration of the vascular features presented by this particular example 

 of early human development confirms one in the idea that the ability of the meso- 

 derm to give rise to vascular endothelium, at least at certain stages and in man, 

 is practically coextensive with the limits of this category of embryonic cells, indi- 

 cating a certain catholicity in tliis respect, As regards free blood-cells, the case is 

 apparently somewhat different. There are, as previously noted, four separate 

 regions in which blood-vessels arise; their connections are secondary, being estab- 

 lished at various times and in a number of chfferent places. These regions are the 

 chorion, body-stalk, yolk-sac, and embryo. To these might be added the amnion, 

 unless one draws the hne between this membrane and the somatopleure so as to 

 leave all the vasofactive material, such as the angiocysts of the umbilical line, well 

 within the embryonic body. The sites of blood-cell formation are most widely 

 distributed in the yolk-sac, while in the body of the embryo they seem to be very 

 sharply restricted (aorta). (Closer phylogenetic relation of the dorsal aorta with 

 the vitelline plexus.) In the chorion there are, at present, no indications of the pro- 

 duction of formed blood elements. 



