HUMAN EMBRYO IN THE SECOND STAGE. 123 



A section of the embryo on a larger scale is shown in figure 58. There 

 appears only the embryonic shield, Sh, which is remarkable for its small area 

 and great thickness. The yolk-sac is also very small and is lined by a distinct 

 layer of entoderm, Ent. Above the embryonic shield is the amniotic cavity, 

 which is, of course, bounded by ectoderm which is continuous with the ectoderm 

 of the embryonic shield. The amniotic cavity has a curious extension into the 

 body-stalk, h. s, by which the embryo is connected with the chorion. The meso- 

 derm is chiefly developed over the chorion, as shown in figure 57. It is very 

 slightly developed in the embryo (Fig. 58, mes), but forms a layer over the yolk- 

 sac and over the amnion, and forms a considerable mass of tissue to constitute 

 the body-stalk, h. s. 



A.mes. 



Atn.c 



Ent. 



Fig. 58. — Embryo of the Preceding Figure More Highly Magnified. 

 Am.c, Amniotic cavity. A.ec, Amniotic ectoderm. A.mes, Amniotic mesoderm, b.s, Body-stalk. Ca, Extra- 

 embryonic ccelom. Ent, Entoderm, mes^. Somatic, mes'' , splanchnic, mesoderm. Sh, Embryonic 

 shield. — {After E. Selenka.) 



Human Embryo in the Second Stage. 



The embryo to be described was investigated by H. Peters. It was found 

 attached to the 'dorsal wall of a uterus almost completely embedded in the mu- 

 cosa, but it was not wholly covered thereby, so that there was no decidua reflexa 

 yet present. A blood-clot was found overlying what would have been other- 

 wise the exposed portion of the ovum. The trophoblast formed an enormously 

 thick layer of very irregular outline and contained many large spaces filled with 

 maternal blood (Fig. 59). The exact external diameter of the ovum could not, 

 therefore, be determined. It measured, however, approximately 2.4 mm. by 

 1.2 mm. The internal diameter of the chorionic vesicle, was about 1.6 by 0.8 

 mm. The trophoblast was everywhere intimately united with the uterine 



