408 A HUMAN EMBRYO OF THE PRESOMITE PERIOD. 



made out. In such plates one can very early recognize that certain cells at the 

 periphery are becoming elongated and flattened into endothelial cells. These shape 

 themselves so as to compactly inclose the more centrally placed cells. It is also 

 characteristic of these angioblastic plates that the cytoplasm of the more centrally 

 placed cells undergoes liquefaction similar to that in the long strands in the chor- 

 ionic villi. The liquefaction of such cells seems to vary, since it is found in some of 

 the smaller angioblastic plates and is absent in some of the larger ones. 



In figure 26 is shown a larger angioblastic mass with less vacuolization and 

 liquefaction of the cells than in the preceding figure. However, endothelial cells 

 differentiating around the contour of the mass can be distinctly recognized. The 

 component cells of the mass show evidences of very active proliferation; they are 

 either in mitosis or in pairs of small, recently divided nuclei. In figure 27 the con- 

 dition is more advanced and a considerable amount of liquefaction may be seen 

 among the more centrally placed cells. In addition to large vacuoles, one finds in 

 such a mass that many of the nuclei are becoming very large and pale, apparently 

 preliminary to their complete disappearance. Although the endothelium can be 

 apparently recognized, it has not yet completely closed off the area from the 

 exoccelom, and even less so from the endoderm. In a few cases completely formed 

 endothelial tubes are found on the yolk-sac, as shown in figure 28. These may con- 

 tain one or more cells with large, round nuclei, but never so many as are present in 

 the angioblastic masses seen in figure 27. We must conclude, therefore, that there 

 is in these cases a considerable conversion of the cellular mass into clear plasma, 

 leaving relatively few complete cells, none which as yet show any evidence of the 

 presence of hemoglobin. 



COMPARISON OF THE MATEER EMBRYO WITH OTHER YOUNG HUMAN EMBRYOS. 



As an aid in placing our specimen in its proper relative position in the series 

 of embryos that have been described in the literature, the more important of these 

 will be briefly reviewed. They will be taken up in the order of their apparent 

 degree of development, which will be determined by the consideration of the mor- 

 phology as well as by the actual size of the embryo and the chorion. The manner 

 of handling and the amount of shrinkage and folding affect the size greatly, par- 

 ticularly as regards the dimensions of the chorion. The disproportion between size 

 and development is even greater where pathological elements have entered. On 

 the other hand, in young stages up to the time of the appearance of the primitive 

 groove, the size of the chorion, owing to its rapid growth as compared with that of 

 the embryo, appears to be a consistent index of the development of the ovum. In 

 older specimens it is necessary to take into account also the morphology of embryo 

 and chorion. 



For the most part the literature dealing with young embryos relates to the 

 histological character of the implantation area and to the interaction between the 

 trophoblastic shell and the uterine mucosa; the structure and form of the embryo, 

 with which we are especially concerned, are given with much less detail. This is 

 due in part to the inadequacy of the material, the chorion and trophoblast being 



