382 Julia B. Platt 
I ask this question, not that I think the vascular system ectodermie, 
but merely to call attention to the fact that even the origin of blood 
corpuscles in the marrow of bone does not prove beyond all question 
their mesodermic origin. 
It is probable that Rast (94) and Harrison (94) were right 
in thinking inadequate methods responsible for a mistake, when 
Kuaarscn (94, 95) derived the scleroblasts of Teleostei from the 
external ectoderm, nevertheless the origin of the skeleton is still 
open to question, since KASTSCHENKO (loc. cit.) believes mesenchymal 
tissues to be the product of cells contributed from all of the germ- 
layers, while GoronowitscH (’92) finds that that part of the mesen- 
chyma which contains the lost cells of the neural crest gives rise 
to bone. Moreover, in a preliminary notice (93) I have maintained 
that the branchial cartilages in Necturus arise from wandering 
eetodermie cells, while v. KUPFFER (95) describes the branchial car- 
tilages in Petromyzon as formed by cells of the deeper layers of 
the ectoderm in situ. The doubt which attaches to the origin of 
the mesoderm affects the chorda in like degree, since like the meso- 
derm, this axial structure, arising from the region where ectoderm 
and endoderm typically unite, seems in higher Vertebrates a product 
of the ectoderm, and of the endoderm in lower Vertebrates. 
v. KÖLLIKER (84) calls attention to the marked difference in 
the kinds of tissue derived from the outer germ-layer. First, there 
are two tissues so fundamentally different as nervous tissue and the 
outer skin, with which the epithelia bounding the cavities of the 
central nervous system might be compared. Further, the medullary 
plate gives rise to a peculiar supporting tissue which is certainly 
not widely different from real connective tissue, and which 
v. KÖLLIKER is inclined to place among the connective tissues, 
I need merely mention the discussion regarding the ectodermic 
or mesodermic origin of the pronephric duct, as enough has been 
said to show that there is hardly an important system of mesodermic 
tissues — excepting only the reproductive glands — which is not 
in one or another Vertebrate, by one or another investigator, attri- 
buted, in whole or in part, to either the outer or the inner germ- 
layer, if not. to both. It is therefore evident that in the term »meso- 
derm«, we have grouped together cells of widely different origin, 
and that we describe as »mesodermic tissues« many tissues which 
are in part at least derived from other germ-layers. If there be a 
middle germ-layer, with our present knowledge, it cannot be defined. 
