STUDIES IN MAMMALIAN BMBRYOLOGY. 383 
is considerably reduced, that it is never a free knob of tissue 
traversing the celome from the embryo towards the serous 
envelope, and that the vascular layer of the chorion represents 
somatic mesoblast of the serous envelope, into which umbilical 
vessels have found their way, and does not represent splanch- 
nic mesoblast of the allantois, which has secondarily spread 
' against a primary and pre-existent parietal mesoblast ; then, 
indeed, it can only create confusion to designate by the same 
name structures that are so differently constituted, although, 
of course, genetically related as are the chorion of man (and the 
Primates) and the outer villiferous wall of the blastocyst of 
the lower mammals. Only in the human chorion there is 
homogeneity between the way in which every part of its sur- 
face is vascularised, it being only the umbilical, and never the 
vitelline vessels which supply it. 
In the lower mammals we hear of an “allantochorion” 
and an “ omphalochorion,” of a true and false chorion and a 
prochorion, and many other names in which the root 
“ chorion” occurs are current in embryological treatises.! If 
1 In order further to convince the reader of the confusing mist of varying 
interpretations by which the name Chorion is more and more obscured, I will 
give a short selection of definitions which we find applied to the name in 
different treatises of embryology. Already by von Baer (‘ Ueber Entwickel- 
ungsgeschichte der Thiere,’ 2e Theil, 1837, S. 53) the difficulties were felt with 
which the exact definition of names of thousand years’ standing are -involved. 
He says :—‘‘ Tausendjahrige Beschreibungen des Chorions [bezeichnen] mit 
dem Namen Chorion eine gefassreiche Haut, die aus der Verwachsung der 
Gefassschicht des Harnsacks und einer gefasslosen aussern Haut des Hies der 
Saugetiiere sich bildet.’’ It was the human chorion that thus traces its 
millennial pedigree, and to this the use of the name should henceforth be 
restricted. The following is the selection above referred to: 
“Man nannte [den Harnsack]im Vogel friiher Chorion”’ (v. Baer, l.c., p.53). 
«Es schwindet [beim Hihnchen] die serdse Hille . . . . die Verbindung 
von der dussern Hialfte des Harnsacks mit der Schaalenhaut [heisst] das 
Chorion ” (v. Baer, l.c., p.55). 
“Das nachste Stadium ..... ist dadurch bezeichnet dan die serése 
Hiille grosstentheils verschwunden ist, und die gefassreiche Allantois jetzt 
die aussere Hihaut, das sogenannte Chorion ausmacht’”’ (Bischoff, ‘ Ent- 
wickelungsgeschichte des Rehes,’ S. 20; cf. Bonnet, ‘ Hihaute der Wieder. 
kauer, 1886, S. 66), 
