372 * MINOT. [Vou. IL. 
discover any intermediate stages. The difficulty of finding the 
first stages of the monster cells indicates that their develop- 
ment must be extremely rapid, almost sudden. 
§ 10. Uterus and embryo at fifteen days and four hours. 
— The swelling of the uterus has considerably increased ; the 
placenta is larger ; the cavity containing the embryo is very much 
larger; the peri-placenta has grown but little. We notice now 
that of the six folds of the uterus, the two placental have ex- 
panded both in width and thickness to a far greater extent than 
the remaining four folds, except that the lateral expansion of the 
two ob-placental folds, by attenuation of their walls, has enabled 
the ob-placenta to occupy an extent of the circumference of the 
uterus which is about equal to that taken up by the placenta 
proper; only about one-sixth of the whole circumference is 
allotted to the peri-placenta. With the naked eye one can see 
that the fissure of the placenta has opened so that the surfaces 
of the two lobes of the placenta now face each other like the 
sides of a V; the surface of each lobe, though somewhat irregu- 
lar, is as a whole arched. The glandular zone is perhaps slightly 
thicker than at thirteen days, but the diameter of the sub-glan- 
dular zone is markedly lessened, owing apparently to the open- 
ing of the interlobal fissure and the consequent flattening of 
the surfaces of the lobes. With a hand-lens one easily recog- 
nizes that the blood-vessels of the vascular zone of the placenta 
are of much greater diameter than at thirteen days, while the 
dissepiments between the vessels are not only relatively but ab- 
solutely thinner than before: this observation does not necessa- 
rily involve the conclusion that there has been an actual loss of 
tissue, for the placenta as a whole has increase in bulk. Let us 
turn now to the microscopical examination. 
The placenta differs but little, except in the respects above 
mentioned, from the stage last described. The mesodermic 
covering of the placenta is well marked, Fig. 12, mes, and the 
foetal mesothelium, mszh, is perfectly distinct ; it leaves the pla- 
centa at its edge to curl over on to the yolk sack, just as at an 
earlier stage, Fig. 8, msth. The side of the lobe next the peri- 
placenta is clothed by ectoderm essentially as described at eleven 
days and partially shown in Fig. 8, ecto; but the ectoderm is 
now more irregular than at earlier periods and is thrown into 
small folds near the point where it is reflected back on the pla- 
