358 
WILLIAM F. ALLFX. 
attained connection with the caudal heart. In two places they have 
gained connection with a mesenchymal space helonging to the embryonic 
lateral sinus, and in one place a direct connection has been established 
with the left lateral sinus. X 100. 
Fig. 11. — Transverse section through the extreme posterior portion 
of the left embryonic caudal heart of a 20 mm. Polistotrema, series 
B. Its exact position is shown by the line 11 in Fig. 6, Note that the 
endothelium of the caudal vein is but little diiferentiated, and the cells 
and their processes cannot be told from the mesenchymal cells imme- 
diately outside that will be concerned in the formation of connective 
tissue and the median cartilaginous bar. Of especial interest is the 
primitive mesenchymal space (c.), which has not at this stage gained 
connection with the caudal vein, but which in a later stage would 
doubtless take part in the formation of the caudal heart. X 225. 
Fig. 12, — Transverse section through the median portion of the 
embryonic caudal hearts of the same 20 mm. einl)ryo as Fig. 11, but taken 
200 microns cephalad ; the exact position of the section is shown by the 
line 12 in Fig, 6, In the left heart note the large ventral and the small 
dorsal mesenchymal spaces, which have attained connection with the 
left caudal vein. Also the posterior tip of another cavity is seen laterad 
of the dorsal cavity, which expands further caudad and terminates in 
the left caudal vein. The embryonic right caudal heart consists of 
several ventral cavities (C.), which unite further caudad and cephalad in 
larger cavities that terminate in the vein. Also where the red corpuscle 
(E. C.) is suspended in the mesenchyme there is evidence of the begin- 
ning of a primitive mesenchymal space. In short, this section shows 
most all of the various stages of the mesenchymal spaces that go to 
make up the caudal heart. At every point the endothelium gives 
evidence of being diiferentiated mesenchyme, and the termination of 
the mesenchymal spaces in the caudal vein have the appearance of 
acrpiiring secondary connections with it rather than out-budding from it. 
X 225. 
Fig. 13. — Transverse section through the extreme anterior end of the 
left embryonic caudal heart of the same embryo as Figs. 11 and 12, being 
taken 120 microns cephalad of Fig, 12, and its exact position in Fig. 6 is 
shown by the line 13, Observe that the orifice of cavity (C.) has the 
appearance of having accpiired connection with the vein by the breaking 
down of the wall separating them. Also what I take to be the first 
mesenchymal space of the ventral veno-lymphatic trunk (?, r. r. (d) is 
shown in cross-section. It is full of red corpuscles, and the loose mesen- 
chyme above is possibly indicative of the place where the first or anterior 
communication with the caudal heart will occur. A comparison of the 
endothelium of the various regions with mesenchyme is of interest.- 
X 225. \ 
