No. 3.] UTERUS AND EMBRYO. 405 
even so recent a writer as Ruge, 129a, 123 and 130, has advo- 
cated, cannot be maintained. Of course, there may be a deposit 
of blood fibrine (coagulum), but it would be pathological, and 
therefore to be distinguished from the normal fibrine of ecto- 
dermal origin. Moreover, the microscopic appearance of a blood 
clot or thrombus is so extremely characteristic that one can 
readily distinguish it from the placental canalized fibrine. 
The ectoderm of the villi of the placenta differs from that of 
the chorionic membrane in several respects: 1°, the cellular 
layer after the first month becomes less and less conspicuous, 
and after the fourth month E 
is present only in a few iso- 
lated patches, known as the 
Zellknoten, and carefully de- 
scribed by Langhans and 
Kastschenko; both of these 
authors were impressed by 
the resemblance of the cells 
to those of the decidua 
serotina; Langhans con- 
cludes that the Zel/knoten 
arise from the serotina, but 
Kastschenko, having traced 
their development from the Cut 23. — Cross-section of a villus from a 
chorionic epithelium, denies placenta of seven months; three blood-vessels 
his predecessor’s conclu- are shown; a,a, thickenings of the ectoderm; 
sion, but still clinging to ane We a thickening transformed into canalized 
, i ; fibrine. X 222 diams. 
idea of a genetic connection 
between the Ze//knoten and the decidua, reverses the reasoning, 
and concludes that the decidual cells arise in part at least from 
the Knotenx. Neither of these authors have found the intermedi- 
ate forms between the two types of cells, and when we examine 
their descriptions critically we find that they have really no 
evidence except the likeness of the cells to offer in favor of 
their genetic relationship, and accordingly Langhans expresses 
himself with characteristic caution. To me the resemblance 
appears altogether superficial; hence my conclusion that the 
Zellknoten are remnants of the cellular layer. 2°, For the most 
part the villi remain covered by the nucleated protoplasm, which 
in many places is thickened. In the later stages these thicken- 
