TAIL-KEGION OF POLISTOTR IGMA (lIDELLOSTUM a) STOUTl. 337 
Ij'mpliatic trunk, but conspicuous right and left valv^es (Fig. IG, 
Val. ( 1 )) will be seen guarding the first or anterior orifice, 
and similar valves are in the process of formation at tlie 
opening of the second or posterior communication. The 
reason why these valves are not shown in reconstruction 8 is 
that the reconstruction is made through the median line of 
the heart, and it would therefore pass directly between the 
rigid and left valves. 
Concerning the histogenesis of the layers of the heart at 
this stage, most any section — as, for example. Fig. IG— will 
demonstrate that the endothelium is well differentiated ; 
instead of consisting of many rounded cells with ndher short 
processes, as in the earlier series, we have here the charac- 
teristic flattened cells and long processes of the adult. Also 
the outer connective-tissue layer has changed from a concen- 
trated mass of mesenchyme to a layer of fairly well differen- 
tiated fibrous connective tissue (Fig. 16, F. C. T.), and the m. 
cordis caudalis (Fig. 16, m. c. c.), although still in an embryonic 
state, is sharply marked out from the connective-tissue layer 
of the heart and the outer undifferentiated mesenchyme. 
85 MM. POLISTOTREMA SeRIES. 
No figures are given to illustrate the condition of the 
caudal hearts in this series, which have attained an adult 
state, except for the extreme caudal end, which bears a 
striking resemblance to the 60 mm. hearts described above. 
The valves and the different layers of the hearts stronglv 
resembled the 20 cm. adult, and one intersegmental vein was 
found emptying into the posterior end of the left heart. 
Since the rate of growth of the Polistotrema embryo is 
very slow, the time consumed from the beginning of the 
development of the caudal hearts in the 20 mm. embryo to 
their completion in the 85 mm. embryo, or a little later, 
requires, probably, several months. This, together with the 
primitive state of the lymphatics in the adult, should make 
Polistotrema a most favoured vertebrate for the onto- 
