16 
AN EARLY HUMAN OVUM 
only distinguishable histologically from the necrotic zone of the decidua 
by being devoid of nuclei. 
This depression is clearly the mouth of the space in the decidua, but 
there is no direct evidence to show whether it represents the point of 
entrance of the ovum, i.e. the point where it first began its destructive 
action on the decidua, or the closed mouth of a fissure into which the 
ovum had been received. This problem will be discussed in a later section. 
TEE TROPHOBLAST. 
The term trophoblast, i.e. trophic epiblast, will be used here in the 
original sense in which it was employed by Hubrecht, to designate that 
part of the ectoderm which does not share in the upbuilding of the 
embryo, or of the amnion in the human subject, but only in the attachment 
and nourishing of the ovum. The trophoblast includes the whole thick¬ 
ness of the wall of the blastocyst, and is differentiated into two parts— 
the Cyto-ti'ophoblast or cell layer, in which the cell outlines are more or 
less preserved, and the Plasmodi-trophoblast or plasmodium, in which they 
are wholly lost. 1 
THE PLASMODI-TROPHOBLAST. 
The plasmodi-trophoblast forms an extraordinarily extensive spun-out 
investment for the ovum. It occurs in masses,. bands, or threads. It is 
difficult to differentiate, in places, the fine threads from fibrin filaments. 
The plasmodial masses are distinguished by the dark, slightly rusty-red 
tint with which they stain, forming a sharp contrast with the blue-pink 
of the immediate wall of the blastocyst, and the red-pink of the necrotic 
layer of the decidua (Plate hi, Fig. 3). The nuclei differ from those 
of the cyto-trophoblast in respect that they are invariably small and stain 
darkly. This latter character is due to the finely granular nature of 
1 The terms cytoblast and plasmodiblast were suggested by Van Beneden. The terms used 
in the text are-those now employed by Hubrecht. In so far as there persists a certain remnant 
of doubt regarding the origin of the plasmodium in the human ovum, the use of these terms 
at this point of our inquiry involves in some sense a petitio principii. 
