A HUMAN EMBRYO OF THE PRESOMITE PERIOD. 



395 



much constricted in this section and consists of 

 only a few cells which are much less compact 

 than in the adjacent sections. Several endo- 

 thelial spaces can be recognized in the compact 

 portion of the body-stalk. A distinct meso- 

 dermal membrane incloses the ventral half of 

 the stalk on each side, spreading over the amnion, 

 whence it continues down over the yolk-sac, 

 constituting the outer of the two layers of the 

 wall of the latter. In the dorsal half of the wall 

 it is distinct from the endodermal layer; ventral 

 to this the two closely fuse and can no longer be 

 distinguished as separate layers. The amniotic 

 ectoderm fits closely against the round ventral 

 surface of the body-stalk and laterally extends 

 downward to a point where it becomes con- 

 tinuous with the transitional portion of the 

 embryonic plate. The embryonic plate proper 

 shows a sharply cut primitive groove, at which 

 point the plate fuses with the endoderm of the 

 yolk-sac. Whether any mesoderm is interposed 

 in this section can not be definitely determined. 

 Lateral to this point there is a considerable 

 amount of mesodermal tissue intervening 

 between the embryonic plate and the yolk-sac, 

 being everywhere closely adherent to the former. 

 It is connected with the yolk-sac by a few 

 slender strands which mark off a series of clear, 

 round spaces, the most lateral of which is con- 

 tinuous with a cleft separating the mesoderm and 

 endoderm from the dorsal portion of the yolk- 

 sac. No endothelial spaces seem to be present 

 in this region. 



Section 63 : The compact portion of the body- 

 stalk is still farther removed from the chorionic 

 membrane than in the previous section. The 

 allantoic stalk is now somewhat larger, but no 

 lumen can be recognized. A small, endothelial- 

 like ring of cells lies free in the exoccelom lateral 

 to the mesodermal membrane covering the body- 

 stalk. This section passes through the amniotic 

 cavity in a transverse direction favorable for 

 showing the structure of its ectodermal walls. 

 Three distinct regions can be made out the 

 flattened amniotic ectoderm, the transitional 

 lateral embryonic plate (consisting of one layer 

 of cylindrical cells), and the embryonic plate 

 proper (consisting of two or three layers of 

 cylindrical epithelial cells). At the primitive 

 groove the ectoderm is in contact with the endo- 

 derm of the yolk-sac. Lateral to this point there 

 is a considerable amount of mesodermal tissue 

 which has the appearance of flowing out from 

 the lateral portions of the embryonic plate. 

 Strands from this mesoderm extend out to the 

 endoderm of the yolk-sac, outlining spaces 

 similar to those described in the last section. 

 The wall of the yolk-sac in its dorsal half con- 

 sists of two distinct layers mesoderm and endo- 

 derm. More ventrally the two layers fuse, and 

 in the extreme ventral pole there would appear, 

 in places, to be only one layer, endoderm. At 



a few points on the ventral portion of the yolk- 

 sac there may be seen clusters of 3 or 4 nuclei, 

 which possibly represent beginning angioblasts. 



Sections 63 and 64: In the body-stalk and in 

 the loose mesodermal tissue between it and the 

 chorionic membrane are several endothelium- 

 lined spaces. In section 64, in the center of the 

 body-stalk, can be seen a well-defined allantoic 

 stalk containing a lumen. The form and struc- 

 ture of the amniotic cavity and its walls are much 

 the same as in the previous section. The rela- 

 tion of the embryonic plate to the yolk-sac is 

 very intimate in the region of the primitive 

 groove. The embryonic plate shows the pres- 

 ence of numerous division figures. The meso- 

 derm intervening between the plate and the 

 endoderm of the yolk-sac is closely adherent to 

 the former. A more detailed drawing of this 

 section is shown in figure 13, plate 3. 



Sections 65 and 66 : In the loose tissue between 

 the body-stalk and the chorionic membrane is an 

 elongated space, in the lumen of which are a few 

 cells. This is the largest space thus far encoun- 

 tered. The lateral surfaces of the compact por- 

 tion of the body-stalk are entirely walled in by 

 a mesodermal membrane. In the region of these 

 sections the allantoic stalk is interrupted; at 

 a point where it should be present one sees only 

 the same mesodermal tissue found in other parts 

 of the body-stalk. The amniotic cavity is 

 rapidly contracting; its apex remains flattened 

 in conformity to the ventral contour of the body- 

 stalk. The embryonic plate is much narrower 

 and the primitive groove is still sharply cut. 

 The ectoderm at this point is not so closely 

 adherent to the endoderm of the yolk-sac as in 

 the previous sections. The ventral portion of 

 the wall of the yolk-sac is veiy much thinned 

 out, and one can not be sure that it consists of 

 more than one layer. In the dorsal portions, 

 however, an outer mesodermal membrane is 

 sharply set off from the endoderm. 



Sections 67 and 68: In the body-stalk is an 

 open space in the area where one would expect 

 to find the allantoic stalk, but otherwise there is 

 no trace of that structure. The amniotic cavity 

 is further contracted (see fig. 12, plate 2). The 

 embryonic plate forming its floor still shows the 

 characteristic outlines of a primitive groove at 

 the center. It is thinner than in the preceding 

 sections, consisting of one to two layers of cells, 

 and the middle is no longer in contact with the 

 endoderm of the yolk-sac. The mesoderm 

 between the embryonic plate and the yolk-sac 

 is predominantly adherent to the former, being 

 separated from the latter by a series of spaces 

 similar to those described in the previous sec- 

 tions. In the mesoderm of this region there is 

 no evidence of blood-vessel formation. To the 

 left of the loose tissue, in section 68, intervening 

 between the body-stalk and the chorionic mem- 

 brane is a group of mesodcnnal cells which take 



