A HUMAN EMBRYO OF THE PRESOMITE PERIOD. 403 



verse sections, both at the caudal and rostral ends, the plate consists of stratified 

 ectoderm about four cells thick and measuring 0.015 mm. midway between the 

 median line and the lateral margin of the plate. Apparently this thickness is fairly 

 uniform throughout the plate, although in many of the sections, owing to their 

 obliquity, it would seem to be much thicker. 



Laterally, along the margins of the plate, the ectoderm folds backward to 

 become continuous with the amnion. At this point there is a transitional area con- 

 sisting of ectoderm a single cell thick, which resembles the ectoderm of the embry- 

 onic plate more than that of the amnion. Since we have no knowledge as to exactly 

 how much of the embryonic plate enters into the formation of the medullary plate, 

 we can not as yet be sure of the destiny of this transitional area whether to allot 

 it to the amnion or to the integument of the body of the embryo. It would seem 

 probable that it might be considered as an area of very active growth of the amnion. 

 This would mean that the area of amnion formation is most active around its 

 margins. 



The primitive groove can be seen very clearly in the sections (figs. 12, 13, and 14, 

 plates 2 and 3), and can be traced into the most caudal sections. Where one would 

 expect to find the primitive node the sections become very oblique, so that it can 

 not be outlined with great certainty. It is probably represented, however, in figure 

 14, plate 3. In this region the ectoderm fuses more or less completely with the 

 endoderm, and lateral to the point of fusion can be seen a flattened area of meso- 

 derm (fig. 13, plate 3). This mesodermal tissue is in the form of a reticula syncy- 

 tium, which in most regions is closely attached to the ventral surface of the embry- 

 onic plate. It is somewhat more loosely attached to the endoderm by irregular 

 trabeculse. In its more lateral areas it is slightly more condensed into a tissue from 

 which are derived the somites and the more laterally situated unsegmented mesoderm. 

 As to the presence of a head process there seems to be no evidence, although the 

 oblique direction of the sections makes it impossible to rule it out with certainty. 

 The most careful scrutiny, however, fails to reveal any sign of a head-process canal. 



AMNION. 



The amniotic cavity is much flattened and consists of scarcely more than a 

 cleft. In this respect figures 15 and 16, plate 4, are very misleading, owing to the 

 tangential direction of the sections. As has been seen, the amniotic ectoderm is 

 directly continuous with the margins of the embryonic plate through a transitional 

 zone. The amniotic ectoderm consists of a single layer of flattened cells which 

 extends to the margin of the body-stalk, fitting tightly against its ventral border 

 and conforming to its shape. This membranous layer of ectoderm is supported by 

 a thin layer of parietal mesoblast, which is also somewhat membranous in character. 

 On reaching the body-stalk the parietal mesoblast passes laterally, so as to par- 

 tially inclose it. Made up of these two layers, the amnion forms a very thin mem- 

 brane which lies in the narrow space between the chorionic membrane and the 

 embryonic plate. Over the greater extent of its dorsal surface many loose strands 

 of mesoderm partly attach it to the chorionic membrane. In figure 5 the cut edges 

 of the amnion can be seen along the margins of the embryonic plate. 



