328 
WILLIAM F. ALLFX. 
mesenchyme, not at this stage differentiated into flattened 
endothelium. 
In certain areas, as may be designated by the red corpuscle 
(Z?. (7.) in Figs. 12 and 13, there are places which appear to 
be the very beginnings of the vacuolation of the mesenchymal 
spaces just described. This process as shown from the 
examination of a number of isolated mesenchymal spaces is 
apparently nothing but a degeneration or atrophy of certain 
of the mesenchymal cell processes and the thickening of 
others to form the boundaries of the cavit}’ ; while some of the 
enclosed cells may enlarge, become round and develop into 
red corpuscles before the mesenchymal spaces become 
connected with the caudal vein. 
Concerning the mesenchyinal spaces (C.) which have already 
gained connection with the caudal vein, there are several 
distinct stages illustrating different periods of growth of the 
larger cavities that take such a conspicuous part in the ex- 
cavating of the caudal hearts out of the mesenchyme. Of 
these an early, middle, and a late stage will be described with 
considerable detail. All of these stages are shown in outline 
in the reconstructions 6 and 7, while some of them may be 
seen to advantage in transverse sections. 
The earliest stage of one of these connected mesenchymal 
spaces (G.) is well illustrated by the dorsal cavity to the right 
in Fig. 12. When compared to the isolated mesenchymal 
space (c.) in Fig. 11 it will be found to be identical, except 
that the mesenchymal lining or undifferentiated endothelium 
of the vein has broken down, joining the cavity with the vein. 
Observe in Fig. 12 some of the remains of the original parti- 
tion at the orifice of the mesenchymal space. The character 
of this orifice and all similar mesenchymal space orifices 
opening into the caudal vein is strongly against the In’po- 
theses that these spaces are out-pocketings or sprouts from 
the caudal vein, as is also the character of their walls. 
Immediately to the left of the above described mesenchymal 
space in Fig. 12 there is a slightly larger but identical con- 
nected mesenchymal space shown, only in this section its 
