MUSEUM OF COMPARATIVE ZOOLOGY. 121 
originates prior to the formative tissue, and in a very different manner ; 
that it has already given rise to the segmental duct, and that the dorsal 
blood-vessel and contained blood corpuscles have taken their origin from 
it before the period is reached at which the formative tissue arises from 
the protovertebra. According to Ziegler’s account, the protovertebrae, at 
the stage in which the formative tissue arises, already show differen- 
tiation of muscle fibre. In Fandulus these two processes also occur 
during the same period (fbr. mu., Plate VI. Figs. 43 , 44), but subsequent 
to the formation of the intermediate cell-mass. It is true, however, that 
a certain portion of this cell-mass still remains in an indifferent state at 
this stage, and its loosely disposed cells are brought into juxtaposition 
with those of the formative tissue, and are identical with them in 
appearance. 
If Ziegler's comprehensive definition for Bildungsgewebe be employed, 
all migatory cells which originate from the mesoderm, of whatever source, 
must be included in it; hence those originating from the intermediate 
cell-mass, and indeed the entire cell-mass itself (excepting perhaps its 
lateral portion, which gives rise to the segmental duct, whose cells in the 
anterior trunk region have from the beginning more or less of an 
epithelial character) are also embraced under this term ; le: ^ inter- 
mediate cell-mass is Bildungsgewebe. Ziegler says ('87, pp. 646, 651) 
that the intermediate cell-mass can at no time be distinctly oisi from 
Bildungsgewebe. This statement is superfluous, unless it is intended 
to mean that the formative tissue (migratory cells) which arises from 
the intermediate cell-mass is identical with that arising from other 
sources (the lower margin of the myotomes) ; but with this construc- 
tion, the statement is true only so far as the appearance of the cells 
and their migratory condition are concerned, the sowrce and fate are 
different. 
I believe that the origin of the “blood-spaces” in Proteus (Wieders- 
heim, 790) has not been traced through earlier stages ; but it seems to me 
very probable that they represent the intermediate cell-mass of Teleosts, 
and that the bi-lobed condition of the cell-mass in the Salmon is here a 
more permanent one, and that the segmental ducts, instead of being 
formed in the lateral region of the intermediate cell-mass, as in the case 
of Fundulus, are here developed in a less peripheral region. It appears 
that the segmental ducts, which at an earlier stage lie in a central region 
of the “ blood-spaces,” later come to occupy a position entirely outside 
of them, but near their lateral boundary, as in the case of the segmental 
ducts in Teleosts. In Proteus the bi-lobed cell-mass, or the “ blood- 
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