No. 3.] OTERUS AND EMBRYO. 349 
(400-500 diams.). The observation of the threads of this net- 
work has led certain investigators to assume the presence of 
connective tissue fibrilla. Scattered about in the connective 
tissue are a not inconsiderable number of leucocytes, 4, 4 /, 
easily recognized by their size and shape, their granular appear- 
ance, deep staining and characteristic nuclei. Around the 
blood-vessels is the perivascular layer of decidual cells, per v, 
which have already been amply described by Masquelin and 
Swaen, Ercolani, Godet, Creighton, and others. Ercolani’s 
descriptions, of which the most important to us is that of the 
rabbit at fifteen days,! 89, p. 278, is far from sufficient. Go- 
det’s paper I know only from an unsatisfactory abstract. 
Creighton’s account of the perivascular layer in the Guinea pig, 
77a, 544, is also good, and he agrees with the Belgian authors 
in tracing the origin of the cells to the metamorphosis of the 
connective tissue. Ercolani opposed this view and maintained 
that the uterine mucosa is completely destroyed, leaving the 
whole placental tissue of the mother to arise as a new formation. 
My preparations render it impossible to agree with Ercolani, 
since they show all the phases of the metamorphosis. It is only 
necessary to follow, in Fig. 3, the three series of cells numbered 
I, 2, 3, 4, each, and to find in all parts of the placenta the same 
appearances; to see the perivascular layer at six and seven 
days, before it is much differentiated; and finally to see the 
perivascular accretions at later stages, to render inevitable the 
conviction that the perivascular layer is modified connective 
tissue. Neither at this stage nor at any earlier or later one 
have I been able to detect any evidence whatsoever of the re- 
sorption of the connective tissue affirmed by Ercolani, $9. 
Masquelin and Swaen describe multinucleate cells, but I fail 
to find them until later stages. 
The blood-vessels have their endothelial lining code 
thickened, each cell for itself, and to its individual degree, Fig. 3, 
Endo; they are stained by alum-cochineal and eosine more 
deeply than the adjacent decidual cells, from which they are 
sharply distinguished. I am unable to recognize any cells which 
might be interpreted as intermediate stages between the endo- 
thelial and decidual cells, as we should anticipate, were Erco- 
1 The specimen described by Ercolani I consider to have been probably really only 
about thirteen days. 
