No. 2.] THE MESODERM AND MESENCH YM. 361 
sem Mesenchym auftretenden Lacunen miissen ebenso wie die 
Blutgefasse (welche . . . einer endothelialen Wand vollkom- 
men entbehren) als Pseudocoel betrachtet werden. . . . Indem 
einzelne Zellen des Mesenchyms frei werden und in das Pseu- 
docoel gelangen, bilden sie sich zu Blutkorperchen um” (Kor- 
schelt and Heider, pp. 1289, 1290). In Salpa the heart is of 
mesodermal origin, and the vessels are lined with an epithelial 
intima: “die Blutgefasse entstehen anscheinend als Liicken- 
rdume innerhalb jenes gallertigen Bindegewebes, welches in 
spateren Stadien die primadre Leibeshohle erfiillt” (cdzd., 
p. 1346). 
In the Vertebrata the cavity of the blood-vessels is probably 
archicoelic and their walls mesenchymatic in origin; but the 
present views upon the derivation of the corpuscles and of the 
blood-vessels themselves are very conflicting. 
To recapitulate in regard to the origin of the blood-vessels 
and blood corpuscles: the cavity of the blood-vessels (with the 
exception of the heart in the Gasteropoda) is apparently always 
archicoelic (blastocoelic), and never gastrocoelic nor coelomic. 
The blood corpuscles in most of the animal groups are derived 
from the endothelial lining of the vessels where such membrane 
is present, but where it is absent, from the surrounding connec- 
tive tissue elements. Accordingly, since the walls of the blood- 
vessels may be mesenchymic (Nemertini?, Hirudinea Asteroidea, 
Ascideae, Vertebrata?), or may be mesodermal (Annelida, Holo- 
thuroidea, Limulus, Scorpionidea, Peripatus, Insecta, Lamelli- 
branchiata), so the blood-vessels themselves may be either 
mesodermal or mesenchymic, or both (e.g., Hirudinea). 
The foregoing brief summary of our present knowledge on 
the development of the various body cavities of the Metazoa 
would lead to the conclusion, that particular differentiations of 
this cavity in one group cannot be safely homologized with 
similarly situated cavities in other groups. And the reason for 
this is not far to seek. For, in the first place, the very differ- 
ent modes by which the process of gastrulation takes place — 
a process which seems to become modified by the mechanical 
factors determining the previous cleavage of the egg — induces 
very heterogeneous formation of both archi- and gastrocoel. 
