THE MESOBLAST 
23 
interests of exact reproduction of what the section shows, of an extremely 
delicate reticulum, which is clearly an artefact due to precipitation of the 
fluid basis of the tissue. The coagulation of the tissue has clearly caused 
some contraction of the mesoblast, which has resulted in its withdrawal 
from the centre of the vesicle so as to leave a clear space in which the 
embryonic rudiment is situated. This retraction of the mesoblast has 
caused some degree of tearing of the embryonic rudiment. 
THE EMBRYONIC RUDIMENT. 
In the absence of any spot in the wall of the vesicle which could by 
any possibility be regarded as embryonic ectoderm, and of any thickening 
which could represent the inward projecting embryonic knob, we must 
recognise the embryonic rudiment in two closed vesicles which occupy the 
central retraction space in the mesoblast. There is a larger vesicle and a 
smaller. It is unfortunate that the larger is collapsed and considerably 
torn, while the smaller, though complete, is probably slightly displaced, 
as it is not directly in contact with the larger sac. 
The larger vesicle extends through 24 sections (Figure n, 1-24), and 
therefore measures in this axis *168 mm. The. cells forming its walls are 
small compared with those of the trophoblast, and are cubical rather than 
columnar, but the protoplasm is frayed, and it is evident that the cell 
bodies have suffered considerably from defective fixation. The nuclei are 
rounded and fairly regular, though an occasional flattened nucleus occurs 
in what appears to represent the roof of the cavity. The vesicle hangs 
free in the central space, being definitely attached at one point only, 
where presumably the mesenchymatous tissue was more resistant. 
A reconstruction in wax (Figure hi) makes it quite certain that we 
have to do with a closed but torn and collapsed vesicle with uniform 
walls; there is no distinction between the cells of the roof and those of 
the floor. There is no indication of a passage from the cavity towards 
the surface of the blastocyst. 
The smaller sac extends through six sections (Figure n, 26-31), and 
measures therefore ‘042 mm. The cells forming its walls are more 
flattened than those of the larger vesicle. It is quite certain that though 
