396 



A HUMAN EMBRYO OF THE PRESOMITE PERIOD. 



part in the formation of a structure that will be 

 followed in the next eleven sections. There is 

 so trace of the allantoic stalk. The primitive 

 groove can still be recognized. The ectoderm 

 at this point, however, is not in contact with the 

 endoderm of the yolk-sac. As in previous sec- 

 tions, the ventral portion of the yolk-sac shows 

 very little evidence of being blood-vessel forma- 

 tion. 



Section 69: The amniotic cavity is more con- 

 tracted and still shows the presence of a primi- 

 tive groove. The relation of the embryonic 

 plate to the mesoderm intervening between it 

 and the yolk-sac is less closely maintained than 

 in the foregoing sections. In the ventral portion 

 of the yolk-sac is a distinct group of angioblasts, 

 consisting of a strand of about 12 cells. On each 

 side of the strand the wall of the yolk-sac is very 

 thin. 



Section 70: The group of mesodermal cells 

 referred to in the last section can now be recog- 

 nized as arranged in the form of a membrane, 

 cut tangentially. The compact portion of the 

 body-stalk is much smaller, and the only evi- 

 dence it shows of an allantoic stalk is a doubtful 

 open space. The amniotic cavity is now very 

 small; its ventral floor still has the characteris- 

 tics of the embryonic plate in contrast to the 

 thin amniotic ectoderm of its roof, as can be 

 seen in figure 11, plate 2. Several angiogenetic 

 areas can be recognized in the ventral portion 

 of the yolk-sac. 



Section 71: The body-stalk is now somewhat 

 detached from the loose mesodermal tissue inter- 

 vening between it and the choriomc membrane. 

 It contains the beginning of the main portion of 

 the allantoic stalk and the tip of the amniotic 

 cavity, the floor of which consists of a small 

 group of ectodermal cells projecting ventrally. 

 The ventral portion of the yolk-sac shows a 

 continuation of the angiogenesis referred to in 

 the last section. 



Section 72: Dorsal to the body-stalk can be 

 seen two separate masses, each of which has an 

 average diameter of about the thickness of the 

 chorionic membrane. The one to the left is a 

 continuation of the mesodermic membrane seen 

 in the previous section, and here it can be seen 

 that the mesoderm incloses a solid mass of 

 ectodermal cells of two kinds; a dorsal, paler 

 group, and a ventral, deeply staining group, the 

 two being sharply marked off from one another. 

 The other mass is somewhat less compact and 

 consists partly of mesoderm and partly of cells 

 whose form is better seen in section 73. In the 

 center of the abdominal stalk is the allantoic 

 stalk, sharply marked off and with a clearly defined 

 lumen. Ventral to this is the tip of the amniotic 

 cavity, whose walls are still differentiated in the 

 dorsal amniotic ectoderm and the ventral 

 embryonic plate. The yolk-sac can now be seen 

 at about its greatest diameter, and is spherical 



in outline. Its dorsal third is composed of two 

 separate layers, mesoderm and ectoderm. In 

 the ventral two-thirds the layers are so inti- 

 mately fused that they can not be distinguished ; 

 at the extreme ventral pole they have the 

 appearance of a single layer, although the 

 existence of angioblasts in this region indicates 

 the presence of mesodermal elements. One 

 group of angioblasts consists of a round, com- 

 pact clump of 5 nuclei. The largest group gives 

 the appearance of an elongated oval endothelial 

 space, compactly filled with about 15 nuclei. 



Section 73: The character of the two small 

 masses seen in the previous sections, in the 

 space intervening between the body-stalk and 

 the chorionic membrane, can now be clearly 

 made out. The larger one (to the left) consists 

 of an ectodennic vesicle with an average diameter 

 of 0.1 mm. The dorsal two-thirds of its wall 

 consists of a single layer of flattened cells resem- 

 bling the amniotic membrane seen in the main 

 part of the specimen. The ventral third con- 

 sists of two or three layers of closely packed 

 cuboidal or cylindrical ectodermal cells. Within 

 the lumen is seen a scant amount of colorless, 

 finely granular coagulum. The whole yolk-sac 

 is surrounded by a more or less membranous 

 and loosely attached layer of mesoderm. The 

 other mass is likewise an ectodermic vesicle sur- 

 rounded by a membranous layer of mesoderm. 

 It is completely detached from the larger vesicle 

 and differs from it in that its wall consists of a 

 single uniform layer of cuboidal cells. Not 

 including the mesoderm surrounding it, its 

 largest diameter is 0.05 mm. The diameter of 

 its lumen is not quite half that of the larger 

 vesicle. Proceeding to the main part of the 

 specimen we find no trace left of the amniotic 

 cavity in the body-stalk. The allantoic stalk, 

 cut slightly oblique, can be seen with its lumen. 

 The body-stalk is fairly well closed in by a mem- 

 branous layer of mesoderm. Near its junction 

 with the yolk-sac is a constriction, at the level 

 of which the endoderm of the yolk-sac extends 

 dorsally about half the distance to the allantoic 

 stalk. Upon studying the wall of the yolk-sac 

 one finds the angioblast-formation to be most 

 active at its ventral pole. 



Section 74: The larger ectodermal vesicle 

 seen between the body-stalk and the chorionic 

 membrane is cut in a very favorable plane and 

 its structure can be clearly recognized. It 

 apparently represents an amniotic vesicle with 

 a single layer of thin, flattened amniotic ecto- 

 derm, and a thick floor-plate of cylindrical 

 embryonic ectoderm, the whole being inclosed 

 by a layer of mesoderm. There is no evidence of 

 a primitive streak. The smaller mass, which 

 is probably a diminutive yolk-sac, shows an 

 incomplete lumen in this section. The body- 

 stalk of the principal embryo shows the allantoic 

 stalk, together with an inverted V-shaped mass 



