400 A HUMAN EMBRYO OF THE PRESOMITE PERIOD. 



CHORION. 



When the ovum was dissected out from the uterus no drawings or measure- 

 ments were made of the chorion. It was noted, however, that after being in the 

 fixing fluid, and just before embedding, the ovum formed a flattened sphere. The 

 sections are cut in the equatorial plane and are nearly circular in outline. When 

 reproduced by means of a profile reconstruction they present the form shown in 

 figure 3, which would correspond to a median sagittal section through the whole 

 ovum. This reconstruction was shown to Dr. W 7 illier and it was his impression 

 that the flattening of the ovum, as there represented, is more extreme than the 

 ovum itself actually showed. In case more sections were lost at the time of cutting 

 than the report stated this error would be accounted for. We must therefore intro- 

 duce a reservation as to the accuracy of the length of the polar axis of the ovum; 

 instead of 3.5mm., as given below, it may be nearer 6mm. Measurements taken from 

 the reconstruction and from the individual sections yield the following dimensions : 



Outside dimension of entire specimen 9.0 by 8.0 by 3.5 mm. 



Inside dimensions of choriom'c sac 6.1 by 5.6 by 2.5 mm. 



Length of longest villus (not including cell-column) 0.8 mm. 



Average length of villi 0.4 mm. 



In the process of dissecting out the ovum practically the whole trophodermic 

 shell was included, some of the areas showing transition into decidua. The arrange- 

 ment and form of the villi and of the encrusting trophoderm are shown in figure 

 3. The chorionic membrane seems to be everywhere intact. The cavity contains 

 a coagulum (magma reticule), which in some places takes the form of a compact, 

 finely granular mass, and in others is arranged in finely granular, reticular strands, 

 irregularly meshed. In the sections stained with carmine the magma is barely 

 perceptible, whereas in sections stained with hematoxylin and counterstained with 

 eosin, aurantia, and orange g, this substance is quite conspicuous. 



The chorionic membrane is made up of a mesodermal and an ectodermal 

 layer. The former presents an entirely different picture from the magma just 

 referred to and there is no evident transition between the two. Owing to the dis- 

 tension of the chorionic cavity the mesoderm is everywhere stretched out as a thin 

 layer covered in by the double-layered ectoderm. The mesodermal and ectodermal 

 layers are for the most part of about the same thickness; in some places, however, 

 the mesodermal layer is thicker. The mesoderm can be traced up into the villi, 

 where it forms their stroma, which is in various stages of vascularization. On 

 examination of the ectodermal layer of the chorionic membrane under higher 

 magnification it can be seen to be made up of an inner, cellular layer (Langhans 

 layer) , and an outer syncytial layer, the cell-boundaries of which are less distinct 

 and the cytoplasm much more compact and granular. This picture varies some- 

 what in different portions of the chorion. In some places the two layers are much 

 the same; in others the contrast is quite striking. In some areas the outer syncytial 

 layer shows active vacuolization. Occasionally small syncytial buds are found pro- 

 jecting from the chorionic membrane. The surface of the chorionic membrane is 

 bathed in maternal blood, as is evidenced by the presence of mature blood corpuscles. 



