134 



THE HUMAN EMBRYO. 



pr.s. 



b.s. 



FIG. 78. SURFACE VIEW OF THE 

 EMBRYONIC AREA OF THE OVUM 

 SHOWN IN FIGURE 77. 



Rafflesi, also described by Selenka, will be found instructive. The relations are 

 here similar to those shown in the section just described, although the stage is some- 

 what more advanced, for we see that the amniotic cavity is larger, that the form- 

 ation of the medullary groove has begun, that the 

 ccelom is beginning to appear in the embryonic 

 mesoderm, and that the blood-vessels of the yolk-sac 

 have increased greatly in size. In this embryo there 

 were traces of the formation of three segments a little 

 in front of the neurenteric canal which was still 

 present and open. This embryo was found to be 

 attached to the wall of the uterus and to be 

 enclosed in a decidua reflexa. In later stages the 

 decidua reflexa of the gibbon unites with the decidua 

 vera, and is then lost completely by resorption. The 

 general character of the ovum and its relations to 

 the uterus justify us in the belief that it is extremely 

 similar to the human embryo at the same stage. 



The Harvard Embryological Collection contains 

 one very well preserved embryo in the third stage, 



b.s, Body-stalk, neu, Neurenteric ca- 



nal. n^Notochord. pr.s, Primi- Senes 82 5- It is a little younger than the gibbon 

 tive streak. embryo above described. A monograph of this valu- 



able specimen is in preparation. 



Human Embryo in the Fourth Stage with the Medullary Plate. 



The general relations in this stage have been indicated by the diagram (Fig. 

 69). A more exact idea of the embryonic structures may be gathered from figure 

 79, a dorsal view of the embryo, from figure 80, which represents a median sec- 

 tion of the embryo taken from a wax model reconstructed from the sections, and 

 figure 81, a transverse section through the neuropore. The general disposition of 

 the parts agrees very closely with the previous stage as described for primates. The 

 embryo and yolk-sac are very small in comparison with the entire ovum, and they 

 are connected by means of the body-stalk, b.s, with the chorion, Cho. The body- 

 stalk contains the entodermal anlage, All, of the allantois. The embryo is covered 

 by the amnion, Am, which arises in front of the head of the embryo, now becoming 

 marked off, and runs above the embryo to join the distal end of the body-stalk. 

 The opening of the yolk-sac, Yk, is about equal to the length of the embryo. The 

 yolk-sac is, of course, lined by entoderm and has a thick layer of mesoderm sup- 

 plied already with relatively large blood-vessels containing blood-corpuscles; the 

 vessels are developed chiefly upon the inferior hemisphere of the yolk-sac. The 

 embryo measured i . 54 mm. in length. Its dorsal surface is represented in figure 

 79. This surface is occupied by the very broad medullary plate of thickened ecto- 

 derm. Toward the middle of its length the medullary plate is somewhat narrower 



