A HUMAN EMBRYO BEFORE THE APPEARANCE OF THE MYOTOMES. 119 



between the cavities of the amnion and allantois, but their epithelial cells fuse into 

 a single mass over a small area of contact. The lumen of this amniotic diverticu- 

 lum, which is also very short, is only a few microns in diameter, tapering slightly 

 toward the allantois. This structure may well be compared with the secondary 

 connection set up between the same cavities in certain reptiles, the canalis amnio- 

 allantoideus of Strahl (Schauinsland, 1902). 



Histologically the amnion is composed of two layers of cells which are generally 

 frankly squamous. The transition of the ectodermic cells to the flattened type is 

 as a rule quite abrupt. In certain places, however, the cells, near the attached 

 border of the amnion, are cubical and become squamous only at some distance from 

 the line of reflection. The cells of the mesodermic layer of the amnion have slightly 

 smaller, more densely staining nuclei and seem to present a clean, even surface to 

 the exoco?lom. Throughout most of the membrane its two layers are in close 

 contact, often almost indistinguishable, but along the borders there is frequently 

 a considerable space between the two, across which run numerous irregular cell- 

 processes connecting the ectoderm and mesoderm but apparently belonging rather 

 to the latter. Scattered mesodermic cells in the interval between the layers are quite 

 common. Posteriorly, where the membrane is cut tangentially, the ectodermic 

 elements appear polygonal in outline, with large, pale nuclei which almost fill the cell. 



In the consideration of the embryo proper we shall begin with the most poste- 

 rior structures and gradually work forward. In like manner we shall endeavor to 

 separate the following observations from the speculations which they invite. 



Mention has already been made of the indications of a canalis amnio-allan- 

 toideus in the caudal extremity of the amnion. A short distance in front of this, 

 in the axis of the blastoderm and at the posterior end of the primitive streak, lies 

 the cloacal membrane. As shown in figures 2 and 3, it measures about 0.12 mm. in 

 length. This measurement and likewise the figures are at best approximations, 

 maximum limits in any case, since it is difficult to determine the exact line, if there 

 be one, between the membrane and the primitive streak. There is in the cloacal 

 region a well-defined groove in the ectoderm, less conspicuous, however, than the 

 primitive groove with which it is directly continuous. In certain sections it is 

 quite evident that the ectoderm and entoderm are in immediate contact, the meso- 

 derm being at some little distance. In other sections, largely on account of the 

 irregular lower surface of the ectoderm, the picture is very much like that of the 

 primitive streak. The conditions found here in the cloacal membrane are such 

 as would be expected from the gradual and not entirely regular transformation of 

 the streak into the membrane. All that is required is an arrest of mesoderm forma- 

 tion and the subsequent separation of the upper and middle germ-layers. The 

 entoderm below is a perfectly distinct layer the cells of which have nuclei larger and 

 paler than those of the other layers. The condition of the ectoderm is such that the 

 real character of its cells can not be made out. Its free surface is of course distinct , 

 but the lower surface is often markedly irregular and frayed out. In the region 

 under discussion at present it is undoubtedly of the columnar type, in most places, 

 if not everywhere, pseudostratified with one, two, or occasionally three layers of 



