394 



A HUMAN EMBRYO OP THE PRESOMITE PERIOD. 



marked contrast to the mesoderm. The transi- 

 tion from the amnion to the embryonic plate is 

 clearly shown on the left side of the section, the 

 transitional portion being made up mostly of 

 one layer of ectodermal cells. The embryonic 

 plate is everywhere clearly separated from the 

 yolk-sac by the intervening mesoderm, which at 

 several points seems adherent to it. 



Section 54: In this section the amnion comes 

 in contact with the chorionic membrane. The 

 amniotic ectoderm does not show any connec- 

 tion with the chorionic epithelium. 



Section 55: In the body-stalk there is seen an 

 endothelial-like space within the lumen of which 

 a cluster of 7 nuclei projects. The mesoderm 

 between the embryonic plate and the yolk-sac 

 is more closely attached to the former than to the 

 latter. 



Section 56: (Compare fig. 16, plate 4.) This 

 section is particularly good for showing the rela- 

 tions between the amniotic membrane and the 

 mesoderm. The former is nearly everywhere 

 cut in transverse section except at its extreme 

 tip. The mesoderm is arranged as a membrane, 

 closely investing the amnion and extending a 

 short distance on the body-stalk. Ventrally it 

 extends downward to inclose the yolk-sac, where 

 it can be traced as a separate lamina for about 

 one-half the distance to the ventral pole. Lying 

 free in the exoccelomic space, at the junction of 

 the amnion with the body-stalk, is a small, 

 empty, endothelial cavity. This can be traced 

 only through two sections. It is surrounded 

 solely by finely granular coagulum. The meso- 

 derm between the embryonic plate and the 

 yolk-sac is adherent at many points to both. 

 The wall of the yolk-sac is cut obliquely for the 

 most part. Its cavity now appears somewhat 

 larger than the amniotic cavity. No blood- 

 islands are seen. 



Section 57: A very intimate relation exists 

 between the lateral wings of the embryonic 

 plate and the subjacent mesoderm. In the 

 body-stalk, near the tip of the amnion, there is a 

 small mass of cells which apparently are ecto- 

 dermal and may represent a but! from the 

 amniotic ectoderm, which appears detached on 

 account of the tangential direction of the sec- 

 tions. 



Section 58: According to the memoranda 

 obtained from Dr. Willie r, two sections through 

 the embryo were lost. On account of the abrupt 

 transition between sections 57 and 58, it would 

 seem probable that the sections are missing at 

 this point. The abruptness is due partly 

 also to the curve in the longitudinal axis of 

 the embryonic plate, so that the plate is cut 

 in this and the succeeding two sections in a 

 markedly tangential direction. In this section 

 the body-stalk is more condensed than hereto- 

 fore and is fairly well inclosed by a membranous 

 arrangement of the mesoderm. It contains in 



its center the tip of the allantoic duct. At one 

 point there is a slight indication of a lumen. 

 The amniotic cavity has become considerably 

 contracted and conforms in a blunt manner to 

 the form of the body-stalk. An intermediate 

 plate still exists between the amniotic membrane 

 and the embryonic plate. This is the first sec- 

 tion in which a sharp, groove appears in the 

 median line of the embryonic plate the primi- 

 tive groove. At this point the ectoderm, meso- 

 derm, and endoderm of the yolk-sac form one 

 continuous mass, which corresponds to the 

 primitive node of Hensen. The extent of this 

 area is exaggerated, owing to the obliqueness of 

 the section. In the ventral part of the yolk-sac 

 a cluster of cells, apparently representing blood- 

 islands, can be recognized. 



Section 59: (Compare fig. 14, plate 3.) The 

 body-stalk consists of two portions a round, 

 more condensed poi tion surrounding the allan- 

 toic duct, and outside of this a triangular area 

 of looser mesodermal tissue which extends up 

 to unite with the chorionic membrane. In the 

 more condensed portion several endothelial 

 spaces can be seen. The allantoic duct contains 

 a lumen. The formation of endoderm, meso- 

 derm, and ectoderm is similar to that in the last 

 section. What appear to be beginning blood- 

 islands can be seen in the ventral part of the 

 yolk-sac. 



Section 60: This section was cut 40ju thick, 

 otherwise it is much the same as section 59. The 

 body-stalk appears as a very condensed mass 

 and at its center can be seen the allantoic duct 

 with a narrow lumen. The stalk is partly covered 

 with a distinct mesodermal membrane. The 

 amniotic cavity fits close against its ventral 

 wall and is considerably contracted, due to the 

 fact that the lateral intermediate plate lies 

 against the main embryonic plate, thereby 

 reducing the width of the cavity by nearly one- 

 half. Owing to the thickness of the section the 

 details of the fusion between the ectoderm, 

 mesoderm, and endoderm can not be made out. 

 As in the previous sections, however, there is no 

 indication of a neurenteric canal. 



Section 61: The transition from sections 60 

 to 61 appears to be very marked, but is due 

 merely to the thickness of the preceding section, 

 which conceals the change in form which the 

 embryo undergoes at this point. By focusing 

 up and down through the section one can 

 recognize the change from a broad tangential 

 section through the embryonic plate to a thin, 

 narrower, transverse section, sufficient to account 

 for the transition between these two sections. 

 In section 61 the compact portion of the body- 

 stalk is separated from the chorionic membrane 

 by the looser mesodermal tissue referred to in 

 the description of previous sections. The 

 attachment is maintained only by loose strands 

 of mesodermal cells. The allantoic stalk is very 



