140 THOMPSON YATES AND JOHNSTON LABORATORIES REPORT 
as shown long ago by Coste, as outgrowths of the ectoderm only,' and concludes 
by saying, ' there is, in my judgment, no reason left for differing from the conclusion 
that both layers are parts of the foetal ectoderm.' 
As most of the foregoing opinions were based on observations made from ova 
of the second or third week, when the villi are fully formed, it is not surprising that 
so much diversity of opinion existed between different observers, and one is convinced 
that it is only by a careful and minute investigation of the earliest possible ova, that 
a true idea can be formed of the development of these layers. 
Peters, in his monograph, Ueher die Einhettung des menschlichen Eies, 
previously referred to, points out that in the earliest stages the cavitv of the ovum 
is surrounded by a thick cellular layer, the trophoblast. This is made up of one 
complete mass of cells, broken up here and there by more or less large blood spaces. 
Peters shews that these spaces communicate with each other by small connect- 
ing canals, which he has been able to trace in a number of serial sections. These 
spaces are filled with maternal blood, and they may again be seen to communicate with 
the maternal sinuses in the decidua. The innermost cells of the trophoblast are 
cubical in shape, and possess oval or round nuclei. They form a continuous layer of 
cells next the mesoblast, but at this early stage are separated from the connective tissue 
in most places, by a narrow space, giving the impression that the mesoblast has not 
yet followed the epithelial mantle in all its outgrowths. Between this layer and the 
uterine mucosa, the cells are arranged in groups and strands, and correspond in every 
respect to the cells forming the inner layer, except in those places where they are in 
contact with the maternal blood. 
Here they are seen assuming a flattened shape, the nuclei tend to become 
spindle or rod shaped, in some places crescents are seen, and here and there a dense 
mass of nuclei, stained very deeply, giving the appearance under the low power of one 
gigantic nucleus. 
In my own specimen, which has been already described, the trophoblast can 
still be recognized as forming a connecting link between the ovum and the maternal 
tissues. The cells are more polygonal than those described by Peters, but they 
correspond entirely with the inner layer of epithelium of the villi, the ' zellschicht ' of 
Langhans, and are seen to be a direct continuation of the cells forming that layer. 
In some of the villi only a single cubical row of cells is seen below the 
flattened outer layer, but in quite a number, multiple row of cubical cells are 
present. 
It is specially noticeable in this specimen, as in Peters, that the mesoblastic 
core is not yet in direct contact with the epithelium, but is separated from it here 
and there by slight spaces. 
From the consideration of these points we can easily recognize in the outer 
flattened layer of the trophoblast, the syncytium, and in the inner cubical layer, the 
