368 HISTOLOGY 



shows an outpocketing extending into the mesoderm at the future caudal 

 end of the embryo; this is the allantois, which soon becomes a slender tube 

 (Fig. 368, C). The mesoderm in which it is lodged later produces the 

 "body stalk." 



The allantois develops very early in human embryos, being present in most if not 

 in all of the specimens thus far obtained. Possibly there is no allantois in the very 

 imperfect embryo described by Bryce and Teacher (Contributions, etc, Glasgow, 

 1908), and there is uncertainty as to its presence in Peters's embryo (Ueber die Einbet- 

 tung des menschlichen Eies, Leipzig, 1899); but in other very young specimens it is 

 well defined. According to Keibel, the allantois first appears in chicks of about 

 twenty segments; in rabbits of eleven segments; in pigs of four to five segments; 

 and in the apes and man, before any segments have formed. Its very early appearance 

 in human embryos is probably correlated with the rapid establishment of the placental 

 circulation, for the umbilical vessels are primarily the vessels of the allantois. 



In Fig. 368, B and C, the entoderm of the yolk-sac is represented as giving rise to a 

 detached cyst (x). There is a cyst of this sort within the chorionic cavity of the 

 somewhat damaged Herzog embryo in the Harvard Collection, and a smaller detached 

 cyst in the very perfect Minot embryo. (These will be further described by the writer 

 in a subsequent publication.) It is possible that such cysts are of regular occurrence, 

 although destined to atrophy. They may be lodged in a strand of mesoderm extend- 

 ing from the lower pole of the yolk-sac downward to the chorion (Grosser, Anat. 

 Hefte, 1913, Abt. I, vol. 47, pp. 653-686), and they may arise as indicated in the 

 diagrams (Fig. 368). 



As the body cavity develops between the somatic and splanchnic 

 layers of mesoderm, it is at first bridged by strands of mesenchymal tissue, 

 forming the "magma reticulare." These strands become attenuate and 

 break down, so that the yolk-sac is then suspended in a well-defined 

 "extra-embryonic ccelom." This part of the ccelom, although within the 

 embryonic membranes, is outside of the body proper of the embryo, as will 

 appear in the following diagrams. 



The arrangement of the membranes surrounding human embryos of 

 about 2 mm. is shown in Fig. 369, A. The chorion has become covered 

 with branching elevations or villi, which contain a vascular core of chori- 

 onic mesoderm. not shown in the diagram. The body of the embryo is 

 connected with the chorion by the mesodermic body stalk containing the 

 allantois. This has become relatively slender. On one side it is covered 

 by the ectoderm of the amnion. The ectoderm, as in preceding stages, 

 may be divided into two parts. Toward the yolk-sac it is thickened and 

 there it forms the axial medullary tube and gives rise ultimately to the 

 epidermis covering the body. Continuous with this epidermal ectoderm 

 is the thinner portion which lines the amnion, as shown in the figure. 

 The amnion forms a membranous sac attached to the ventral side of the 

 embryo, leaving an aperture through which the yolk-sac projects downward 

 into the extra-embryonic ccelom. The ccelom now extends between the 

 amnion and chorion, except at the narrow body stalk. The yolk-sac has 



