HUMAN EMBRYO IN THE EOURTH STAGE. 



129 



bryonic 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 seg- 

 ments 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 refiexa 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 Yk. - — 



at the same stage. 



Human Embryo in the Fourth Stage with 

 the Medullary Plate. 



The general relations in this stage 

 have been indicated by the diagram (Fig. 

 54). A more exact idea of the embry- 

 onic structures may be gathered from 

 figure 65, which represents a median sec- 

 tion of the embryo taken from a wax 

 model reconstructed from the sections. 

 The general disposition of the parts 

 agrees very closely with the previous 

 stage as described for primates. The em- 

 bryo and yolk-sac are very small in com- 

 parison with the entire ovum, and they 

 are connected by means of the body-stalk, 

 h.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 becom- 

 ing marked off, and runs above the em- 



Am. - — 



Md.gr. -S^-; 



Neu. 



Pr, gr. 



b. s. 



Cho. 



"^0^^ "^ 



Fig. 64. — Reconstruction of a Human Em- 

 bryo 1.54 MM. Long. The amnion has 

 been opened to show the dorsal surface of 

 the embryo. 



y^, Yolk-sac. Am, Amnion. Md.gr, Med- 

 ullary groove. Neu.c, Neurenteric canal. 

 Pr.gr, Primitive groove. b.s. Body-stalk. 

 Cko, Chorion. — [After Count Spec.) 



bryo 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 supplied already with relatively large blood-vessels containing 

 blood-corpuscles; the vessels are developed chiefly upon the inferior hemi- 

 sphere of the yolk-sac. The embryo (Fig. 65) measured 1.54 mm. in length. 

 Its dorsal surface is represented in figure 64. This surface is occupied by 

 9 



