128 A HUMAN EMBRYO BEFORE THE APPEARANCE OF THE MYOTOMES. 



Connections between these last-named structures and the mesothelial ingrowths are 

 not especially in evidence, but we have not gone into a detailed study of them in this 

 respect. The occurrence of similar conditions in the wall of the yolk-sac we would 

 not like to exclude, because the histological pictures are here less satisfactory than 

 anywhere else in the whole specimen. 



III. THE CHORION (MICROSCOPIC) AND EXOOELOM. 



Examination of the sections (plate 1, figs. 3, 4, and 5) shows that the villi in 

 the equatorial zone are much more developed, longer, larger, more numerous, and 

 more richly branched than on the two flattened poles of the vesicle. The meso- 

 dermic portion of the chorionic wall is a thin, fairly uniform stratum, the ragged 

 exoccelomic surface of which is in marked contrast with the same surfaces of the 

 embryonic structures. Within the larger villi, even far from the embryo, are found 

 numbers of open spaces, some of the smaller having a fairly distinct endothelial 

 lining. Some of the unlined spaces in the wall of the sac are very large, and they 

 may be brought about in part or at least accentuated by the pulling away of the 

 mesodermic from epithelial constituents of the chorion which is in evidence almost 

 everywhere. Occasionally short strands resembling the angioblast cords are seen, 

 even in the bases of the villi, but these and undoubted vessels in the villi are by far 

 most frequent near the attachment of the body-stalk. 



The villi possess a loose mesenchymal core which in the shorter ones extends 

 quite to their free ends, while in the longer and larger equatorial villi this core is not 

 so extensive. The inner layer of the epithelial covering of the villi and also of the 

 chorionic wall is made up of distinct cellular elements, polygonal in outline and vary- 

 ing from thick squamous to low cuboidal, constituting the cytotrophoblast on the 

 layer of Langhans. Cell-boundaries are here uniformly distinct, and both the cyto- 

 plasm and the nuclei stain more lightly than the same parts of the overlying syncy- 

 tium. Often the line between the Langhans layer and syncytium is very sharp, 

 again decidedly vague, while in numerous places either layer may be so reduced 

 as to seem the only covering of the mesodermic core. Most frequently it is the syn- 

 cytial layer which is so markedly thinned or apparently absent. The fact that the 

 line between these two layers can not always be seen, and the occurrence in the 

 deeper portions of the syncytium of what seem to be indistinct cell-boundaries, 

 would point to the close genetic relationship of the two layers. Distally the cellular 

 layer of the villi passes over into the cell-columns by means of which the villi are 

 extensively united. This is especially conspicuous in the case of the equatorial villi, 

 among which are also found extensive irregular masses, the trophoblastic cell-islands, 

 which on the surface toward the ovum gradually merge into the cell-columns of the 

 villi. These cell-islands are composed of large, very pale cells with distinct bound- 

 aries and large, pale nuclei. The constituent elements are for the most part irregu- 

 larly polygonal, but they may take on an elongated, spindle-like form, as if actively 

 drawn out. A faint vacuolization is not infrequent. In many places, but most 

 marked in the neighborhood of the embryonic attachment, these cellular masses 

 form practically an inclosing shell over the inter villous spaces beneath. 



