No. 2.] DEVELOPMENT OF THE HUMAN COELOM. 419 
coelom was filled with a clear fluid, and many firm shreds of a 
fibrine-like body which obscured the embryonic vesicle greatly. 
With much difficulty the embryo could be outlined, and these 
drawings proved to be of great service in making the recon- 
struction. The portion of the chorion to which the embryo 
was attached and the embryo were stained in carmine and 
imbedded in paraffin. The whole was cut into sections, at 
right angles to the body, 10 yp thick. 
Every other section was enlarged 100 times and drawn on 
wax plates 2 mm. thick, and from them the model of the 
embryo was made. The model gives the whole central 
nervous system, the entoderm throughout its extent, the blood- 
vessels, and the muscle plates. 
The shape of the neural tube is given in the diagrammatic 
outline. It was closed only along the middle of the body, being 
open in front down to the beginning of the fourth muscle plate. 
From the beginning of the fourth plate to the beginning of 
the fourteenth it was closed, and from there on again it was 
open. In the figure the portions between x and x’ indicate 
to what extent the tube is closed. In Figs. 17 and 18 the 
tube is nearly closed, while in Fig. 20 the tail end of the tube 
is just beginning to separate from the ectoderm. The cephalic 
end of the tube already clearly outlines the fore-brain, the 
mid-brain, and the hind-brain; the constriction, Fig. 16, a, 
indicates the junction between the first two. On the ventral 
side of the fore-brain there are two marked pockets, one on 
either side, just behind the neuropore, which are no doubt the 
primary optic vesicles. It shows that in the human embryo these 
are fully outlined before the brain has separated itself from the 
ectoderm. Farther behind, very near the dorsal median line 
and about in the middle of the head, there is a short pocket of 
thickened ectoderm, the otic vesicle. Towards the hinder 
end of the embryo the spinal cord communicates by means of 
a solid band of cells with the entoderm, Fig. 20. At no point 
in this communication is there a canal, so it must be viewed as 
the last remnant of the neurenteric canal. The location is 
opposite the twelfth muscle plate, or in the neighborhood of 
what will later on be the position of the first rib. The chorda 
