A HUMAN EMBRYO OF THE PRESOMITE PERIOD. 413 



in slightly larger specimens, remains to be determined by a comparison of more 

 material than is available at present. It is quite probable that the form of the 

 vesicle is dependent to a great extent upon such factors as the handling and preser- 

 vation of the tissues and the technique of embedding. In the Minot specimen there 

 is a distinct primitive knot where the ectoderm and endoderm are definitely blended. 

 Numerous blood-vessels are present in the wall of the yolk-sac and also in the body- 

 stalk and the chorion. 



In addition to the two just mentioned, there are three other specimens closely 

 resembling our own. These have been described respectively by Debeyre (1912), 

 van Heukelom (1898), and Giacomini (1898). Of these, the Debeyre ovum is in the 

 best state of preservation. In it the embryonic shield is in the form of an elliptical 

 and dorsally convex plate, consisting (according to the author's description) of a 

 single layer of cylindrical epithelial cells showing numerous karyokinetic figures. 

 In consisting of a single layer of cells it differs from the embryonic shield in the 

 other specimens mentioned, in all of which it was described as stratified. Debeyre 

 explains the appearance of stratification as due to the distribution of nuclei at 

 different levels. There is a well-marked primitive streak, along which exists a close 

 union between the ectoderm and endoderm, and a primitive groove 0.54 mm. long. 

 At the caudal end of this there is a cloacal membrane where the ectoderm and 

 endoderm again join. No trace of a neural groove or neurenteric canal could be 

 found. The amnion incloses a flattened space conforming to the shape of the 

 embryo. There is no amniotic canal present. An allantois 0.4 mm. long is present 

 and has a distinct lumen, but no terminal dilatation. The yolk-sac is lined with 

 endoderm, which has the appearance of a protoplasmic syncytium sown with nuclei; 

 the contours of the cells, for the most part, can not be made out. There are no 

 epithelial buds or glandular diverticula. The supporting mesoderm is extremely 

 variable and at places is entirely lacking. At the ventral pole of the yolk-sac, 

 apparently arising from the mesoderm, are numerous blood-islands in the following 

 forms: (1) Full and consisting of many layers of cells arranged concentrically; 

 (2) an opaque mass of amorphous substance sown irregularly with nuclei of uniform 

 size; (3) cellular elements arranged around a central cavity; (4) uniform cells sur- 

 rounding a cavity which may or may not have partitions and which contain 

 differentiated elements, some having large nuclei with little protoplasm; (5) irregular 

 strands of closely packed mitotic cells. Blood-islands are also present in the body- 

 stalk, but none are to be seen in the region of the embryonic plate. 



In the van Heukelom specimen the chorion is torn and collapsed and the tissues 

 are in a rather poor state of preservation. From its general form and size, however, 

 one can see that it closely resembles the other specimens of this group. In the brief 

 anatomical description mention is made of the presence of the primitive groove, 

 allantois, and blood-islands in the walls of the yolk-sac, and what were possibly 

 blood-vessels in the chorionic membrane. 



The Giacomini specimen is in form very much like the "v. H" ovum of v. Spee, 

 but from the figures and description it is apparent that it had undergone marked 

 maceration. Owing to mechanical injury the yolk-sac is partially detached from 



