'\ THE CHORIONIC VILLI. 345 



tive material vary extremely in appearance. In some cases they are small and 

 stain rather deeply, and are then found to be present in more or less considerable 

 numbers. Occasionally, however, the nuclei are much larger, and more rarely 

 one sees a nucleus of exceptionally great diameter. 



Our knowledge of the human trophoblast being still very imperfect, its full 

 history is still partly a matter of supposition. The appearances described indi- 

 cate that the trophoblast undergoes a rapid degeneration, during which the cells 

 fuse, while their protoplasm becomes a hyaline material. We must then further 

 suppose that the degenerated substance is resorbed and disappears altogether. 

 Finally, we must assume that the entire trophoblast does not disappear, but that 

 enough is preserved to form the permanent covering of the villi. 



It may be noted that the specimen on which the above description is based 

 agrees essentially with the specimen described by Siegenbeek van Heukelom, 

 which is regarded as normal. 



The Chorionic Villi. 



The villi may be obtained in connection with the preparations of the uterus 

 and placenta. In order to see the youngest stages of the first villi it is necessary 

 to have the chorionic membrane of the second or early part of the third week. 

 At this stage the trophoblast is present and the first villi are appearing (compare 

 page 342). To study the growth and form of the villi, single villi or pieces of 

 villi should be snipped off from the chorion at various stages. Such pieces may 

 be examined as opaque objects in alcohol, or they may be stained and mounted 

 as permanent preparations. To obtain injected villi it is best to inject the pla- 

 centa through one of the arteries of the umbilical cord, using as the injecting 

 mass, gelatin colored with carmine or Prussian blue. Such injections are very 

 easily made. 



Branching of the Villi. The formation of a branch is usually initiated by 

 an outgrowth of the ectoderm. Branches grow very rapidly; the outgrowth 

 which forms the branch occurs with every degree of participation of the meso- 

 derm. The two extremes are first the bud, consisting wholly of epithelium, which 

 may become a process with a long, thin pedicle and a thickened free end remain- 

 ing sometimes entirely without mesoderm. Later the mesoderm penetrates it 

 and completes the structure. Second, a thick bud with a well-developed cord 

 of connective tissue and having a nearly cylindrical form. Between these 

 extremes every intermediate stage can be found. The tips of the branches are 

 for the most part free, but some of them come in contact with the surfaces of the 

 decidua and penetrate it for a short distance. By this means the villi of the 

 embryo are attached to the decidua of the mother. The villi do not penetrate 



