The Development of the Cartilaginous Skull ete. in Necturus. 381 
Ragı (’87) confirms the statement for the Amphibia, while Rickert 
(88) affirms that in the Selachii, individual cells migrate from the 
wall of the alimentary canal to take part in the formation of the 
heart and blood-vessels. 
In Necturus, cells which form the endothelium of the heart 
also appear to arise from the endoderm. There are, however, meso- 
dermic cells in the immediate neighbourhood, to which these endothelial 
cells might possibly be traced by one strongly convinced that the 
origin of the vascular system is, or ought to be, throughout meso- 
dermic. However, the ventral blood-vessel (i. e. heart) immediately 
divides into two large vessels that run forwards and upwards to 
the dorsal aorta, through the mandibular arch. In Necturus, these 
mandibular aortae arise as spaces enclosed by thin protoplasmic 
extensions from cells still remaining in the wall of the ali- 
mentary canal (fig. 19, pl. XVIII). That these cells consequently 
are endodermic is not open to question. It is not even »theoretisch 
denkbar« that they have been displaced from the mesoderm at an 
earlier stage, for the axial mesoderm primarily extends in Necturus 
no farther than the transverse plane in which the chorda ends, or 
to the plane of the hyomandibular clefts. Anterior to this plane, 
the ectoderm rests directly on the endoderm with no intervening 
mesoderm. When the mesoderm finally extends forwards beyond 
the plane of the hyomandibular clefts, it does so as a distinct and 
sharply defined tissue, by no means continuous with the endoderm. 
Hence there can be no thought of an earlier fusion between the . 
mesoderm and endoderm in the region in which the mandibular 
arteries arise. Moreover, the cells from which the mandibular arteries 
are formed, are not from the wall of the wide archenteron, but from 
the wall of the anteriorly closed tube of the alimentary canal. 
In giving the grounds for believing the mesoderm to be the 
true source of the vascular system, ZIEGLER (loc. cit.) says: »In the 
Anura and in all Amniota the marrow of bones is concerned in 
the production of red blood corpuscles: no one will doubt that the 
marrow of bone belongs to the mesoderm and, indeed, to the mesen- 
chyma.« Now, according to KASTSCHENKO (loc. cit.), the mesenchyma 
is in part ectodermic, and according to GORONOWITSCH ('92, page 456) 
just this eetodermie part of the mesenchyma contributes in the bird 
to the formation of bone. How then may we be sure that no cells 
of ectodermic origin take part in the formation of the vascular 
system ? 
