A HUMAN EMBRYO OF THE PRESOMITE PERIOD. 



393 



DESCRIPTION OF SECTIONS. 



The first 26 sections of the series include only 

 trophoblast and villi. The 27th, 28th and 29th 

 sections traverse the chorionic membrane. In 

 the 30th section one can see the chorionic cavity 

 or exoccelom. The succeeding 13 sections 

 include the narrow interval between the chorionic 

 membrane and the embryo. This space is filled 

 by a fine granular network of coagulum, in the 

 meshes of which a few maternal red blood-colls 

 can be seen. Since the chorionic membrane 

 was not torn, it is probable that these red cells 

 were displaced during the process of sectioning. 

 The knife may have carried them from the 

 intervillous spaces. 



Sections 44 and 45 pass tangentially through 

 the extreme rostral portion of the amnion. In 

 section 45 the amniotic cavity makes its first 

 appearance. Dorsally it is loosely adherent to 

 the chorionic membrane. 



Section 46 passes through the rostral end of 

 the amniotic cavity and shows the obliquely cut 

 rostral margin of the embryonic plate. An 

 irregular strand of mesodermal cells can be seen 

 everywhere investing the amniotic membrane 

 and embryonic shield. 



Sections 47 to 49 penetrate more deeply into the 

 amniotic cavity, showing the folds of the 

 amniotic membrane. Ventral to the amniotic 

 cavity the yolk-sac makes its appearance. 



Section 50: A more detailed drawing of this 

 section is shown in figure 16, plate 4. The 

 amniotic cavity is still larger than the yolk 

 cavity. The amniotic membrane is folded so 

 that it can be seen in both transverse and tan- 

 gential sections. Where cut transversely it 

 appears as a single layer of flattened ectodermal 

 cells closely invested by an irregular layer of 

 mesoderm. Dorsally, loose strands of mesoderm 

 extend toward the chorionic membrane. Around 

 its lateral margin the amniotic membrane bends 

 sharply to become the embryonic plate. In 

 its more lateral portions the embryonic plate 

 consists of one or two layers of cylindrical 

 ectodermal cells. The nuclei for the most part 

 are toward the bases of the cells. More cen- 

 trally the number of layers is increased to 3 or 

 4. Owing to the tangential direction of the 

 sections, the middle portion of the embryonic 

 plate appears thicker than it actually is. Ven- 

 tral to the embryonic plate, strands of mesoderm 

 extend between it and the yolk-sac. The meso- 

 derm seems to be more adherent to the ecto- 

 derm of the embryonic plate than to the ento- 

 derm of the yolk-sac; between it and the latter 

 there is a series of roomy clefts. The wall of 

 the yolk-sac consists of a rather poorly defined 

 strand of protoplasm with large, round, and 

 oval nuclei, arranged irregularly in two layers. 

 Some of these stain intensely, others are pale. 

 Cell-boundaries can not be well made out. 



Within the yolk-sac there is a considerable 

 amount of finely granular coagulum similar to 

 that in the exoccrlom. The amniotic cavity is 

 perfectly clear. Apparently the outer row of 

 nuclei represents the investment of mesoderm. 



Sections 51 and 52: The space between the 

 amnion and chorionic membrane is bridged by 

 a mass of mesoderm more dense than in the 

 previous sections, so that the sections are now 

 definitely in the region of the body-stalk. Among 

 these cells may occasionally be seen a group 

 arranged in circular formation, so as to form a 

 disconnected endothelial-like space. These spaces 

 are for the most part empty, but now and then 

 they inclose one or more cells. The amniotic 

 membrane is much the same as in the previous 

 sections. The embryonic plate is cut obliquely. 

 The central part is uniform in appearance, 

 showing no evidence of an neurenteric canal. 

 The margin towards the amniotic cavity is 

 covered on each side by a lateral sulcus which 

 demarcates a transitional portion intervening 

 between the embryonic plate and the amniotic 

 membrane. This portion resembles the rhombic 

 lip, to which is attached the tela choroidea in 

 the hind-brain of the adult. About one-third 

 of the distance between this lateral sulcus and 

 the middle line is another groove, less marked, 

 but which seems to be fairly constant throughout 

 the successive sections. In the middle line there 

 is no groove. Between the two sulci on each 

 side the embryonic plate bulges into the lumen 

 of the amniotic cavity, resulting in a longitudinal 

 ridge which can be traced backward to about 

 the region of the primitive groove. In the 

 embedding of the specimen the tissue became 

 brittle and an occasional crack is found crossing 

 the embryonic plate. The mesoderm ventral 

 to the embryonic plate shows pointy of intimate 

 attachment to the latter, particularly in the 

 lateral portions of the plate. Strands of meso- 

 derm cross from the embryonic plate to the 

 yolk-sac, forming trabeculse, between which is a 

 series of clear, round spaces. Laterally, these 

 spaces are continuous with the cleft that inter- 

 venes between the mesoderm and the wall of 

 the yolk-sac, extending about one-quarter of the 

 distance toward the ventral pole. The two 

 layers of the wall of the yolk-sac, the endoderm 

 and mesoderm, are more distinct than in the 

 previous sections. No indication of blood 

 islands is seen in this region. The content of 

 the yolk-sac resembles the granular magma seen 

 in the exocoslom and is perhaps slightly greater 

 in amount. 



Section 53 shows very well the attachment 

 between the amnion and the chorionic mem- 

 brane. The mesodermal cells are closely clus- 

 tered around the amnion. The apex of the 

 amnion is cut tangentially and so stands out in 



