390 MINOT. (Vou. II. 
this course of reading and a study of my own extensive mate- 
rial I have based the following history of the chorion, which 
passes briefly over what is known, and dwells upon what is 
founded on my own observations. 
The human chorion as I have defined it} is “the whole of 
that portion of the extra-embryonic somatopleure, which is not 
concerned in the formation of the amnion.’”’ The human cho- 
rion is remarkable for its very early complete separation from 
the yolk sack, and for its precocious development of villi. Both 
of these developments had already taken place in His’ youngest 
embryo, and in Reichert’s ovum, which is supposed to be nor- 
mal and the youngest known, there were chorionic villi, though 
no embryo was distinguished. Reichert’s description is not 
satisfactory, his long memoir? being principally concerned with 
speculations. 
The villi of the chorion, as shown long ago by the obser- 
vations of Coste, are formed at first only 
by the ectoderm. I reproduce here Fig. 
6; of Pl. IL, ‘xeferring to the “human 
species from Coste’s great work. The 
hollowness of the villi and their clumsy 
shape are to be especially noted. The 
mesoderm grows into the villi subse- 
quently. The branches of the villi grow 
out in a similar manner, the process 
being led, as it were, by the ectoderm. 
Orth, in a special paper, 118, has used 
these facts to argue against Boll’s Prz- 
cip des Wachsthums. Kollman’s obser- 
chorion of ‘an-embryo sup-) “ations ©) on the (prom the ar jvalln dinens 
posed to be about eighteen the fourth week are particularly instruc- 
days; mes, mesoderm; ec, tive. The outgrowth of the branches 
peaeyaet aL mpeg BT very rapid and occurs with every de- 
gree of participation of the connective 
tissue. The two extremes are: 1°, a bud consisting wholly 
of epithelium, which may become a process with a long, thin 
pedicle, and a thickened free end remaining entirely with- 
Cut 72. — Portions of the 
1 Buck’s Reference Handbook, Medical Sciences, — Art. Chorion. 
2 Reichert, Berlin Akad. Abhandlungen, 1873. 
3 Kollman, Arch. Anat. Physiol. Anat. Abth., 1879, 297. 
