No. 3.] UTERUS AND EMBRYO. ae 
entire extent, but the greatest accumulation is where they first 
are developed, directly opposite the placenta. In this region 
(Fig. 17) they occupy not only the connective tissue of the 
mucosa, a, 6, but also the territory of the circular muscular coat, 
where they lie, c, between the bundles, msc, of muscular fibres, 
which they have forced apart to make room for themselves. 
The smallest monster cells, a, are found nearest the lining epi- 
thelium, ef; those at the base of the mucosa, 4, are bigger, but 
the biggest of all are those which lie in the outer part of the 
muscularis, c; if, therefore, the cells arise from the epithelium 
and migrate outwards, they must grow while they move. My 
preparations show in the nuclei of the monster cells certain 
large, deeply stained fragments which are perhaps chromatine, 
Fig. 18. Owing to the stretching of the uterine walls, the 
regenerated glands of the ob-placenta are no longer follicular as 
at thirteen days (Fig. 10), but are again stretched out, so as to 
approximate a second time to the form of shallow, open cups, 
which they had at eleven days (Fig. 6); but where the monster 
cells have accumulated most (Fig. 17), the only distinct trace of 
the glands is the irregularity of the free surface covered by 
epithelium, ef. 
The embryo and its appendages do not show much alteration 
in the parts concerning us in the present article. We may, 
however, note especially two changes in the outer germ layer. 
1°. On the strip of ectoderm between the vena terminalis of the 
yolk sack and the points where the ectoderm joins the placenta, 
there are a number of thickenings, which form small papillz 
upon the outer surface of the layer. These outgrowths are 
solid ectoderm, and like the buds of the villi of the human cho- 
rion contain no mesoderm. Whether these structures do 
become actual villi in later stages, 1am unable to say. 2°. Over 
the yolk sack the ectoderm has become a cylinder epithelium, of 
which the outer surface is irregular, each cell projecting a little 
more or less than its neighbors. A similar modification occurs 
in the opossum according to H. F. Osborn, 61 A, 378-379, 
Pl. XVIL, Fig. 4, and Selenka (Eutwickelungsges. d. Thiere, 
Taf. XXVIIL, Fig. 5). It is probable that the ectoderm 
assumes this modification in other mammals, where it remains 
attached to the yolk sack owing to failure to form a complete 
chorion. 
