22 
AN EARLY HUMAN OVUM 
(4) The plasmodium and necrotic zone of the decidua are not every¬ 
where directly in contact with one another; a union between the two 
occurs only here and there; elsewhere a space is left between the two 
containing red blood corpuscles and leucocytes. There is no indication 
that the wall of necrotic tissue is being absorbed by phagocytosis in the 
strict sense of the term; the appearances suggest rather solution by enzymes 
produced by the trophoblast. 
On the whole we are inclined to conclude that these elements are 
derived from the necrotic zone of the decidua; that they are in short 
decidual cells set free in the process of absorption of the necrotic tissue. 
It must be admitted however that there is no histological criterion by 
which it can be absolutely determined whether they are maternal or foetal 
derivatives. 1 
THE CONTENTS OF THE BLASTOCYST. 
THE MESOBLAST. 
The cavity of the vesicle is occupied by a very delicate cellular 
reticulum, or loose syncytial tissue which has the characters of mesenchyme. 
It represents the earliest stage yet observed of the mesoblast. This 
mesenchymatous tissue shows no signs of cleavage into a parietal and 
a visceral layer; it is not yet arranged in a definite and denser layer 
round the wall of the vesicle, nor are there any processes of it indenting 
the wall. 
The constituent cells of the mesoblast are minute, rounded or stellate 
elements united together by very delicate protoplasmic threads. They 
are brought out in the drawing reproduced in Plate III, Fig. 3, but the 
network which the}^ form is somewhat obscured by the delineation, in the 
1 Since the above was written we have had the opportunity of seeing a demonstration by 
Graf v. Spee of the early phases of placentation in the guinea-pig, at the meeting of the 
Anatomische Gesellschaft in Berlin (April 23rd, 1908). Graf v. Spee showed that in the guinea- 
pig there is a layer of cells of foetal origin outside a plasmodial formation, which, though less 
extensive, has the same character as the plasmodium in our human ovum,—in short, that there 
are three layers of the trophoblast. Until his account of his preparations is published we can 
do no more than point out that Graf v. Spee’s researches may possibly necessitate some modification 
of our interpretation of these elements. 
