The Development of the Cartilaginous Skull ete. in Necturus. 379 
of the organism. Where the two primitive germ-layers pass into 
one another, along the margin of the blastopore, or in the line of 
its morphological equivalent, lies the area of doubt, coextensive 
with the mesoderm which is here proliferated. Not only is the 
degree of specialization which has taken place in the cells of the 
mesoderm open to question on the basis of the ectodermic or endo- 
dermie affinities of the tissues they subsequently produce, the very 
origin of the cells is doubtful, for in the highest Vertebrates the 
cells of the middle layer arise from the ectoderm; in the lowest 
Vertebrates — unless Amphioxus be, as Lworr (94) claims, an 
exception — the mesoderm is derived from the endoderm, while in 
intermediate forms both of the two primitive germ-layers contribute 
to its formation. 
This difference of origin is explained by coenogenetic changes 
which, in separating the two primitive germ-layers, associate the 
band of cells destined to give rise to the mesoderm with one or 
other of the primitive layers according as the line of separation 
passes along the ectodermic or endodermic margin of the band. 
Thus, if the separation of the two germ-layers takes place along 
the endodermic margin of the band of cells from which the mesoderm 
arises, this tissue appears to be derived from the ectoderm, or from 
the endoderm when the conditions are reversed. 
Although the explanation is simple, the complications produced 
by coenogenetic digressions from the plan of typical gastrulation, 
have led to the greatest divergence of opinion regarding the actual 
and theoretical, or phylogenetic, origin of the mesoderm, as amply 
testified in the critical reviews of MEHNERT (91), and KEIBEL (93). 
When one turns from the question of the origin of the mesoderm 
to ask, what are the organs to which the middle layer gives rise? 
The answer is not less indefinite. The mesoderm gives rise to 
muscles. Yes, but v. KOLLIKER (84) tells us that the outer germ- 
layer possibly gives rise also to nonstriated muscle fibres in connection 
with glands of the skin, since, as first maintained by v. KOLLIKER 
(49), and afterwards confirmed by Lrypic, and Ranvier, muscle 
cells are found between the secretory cells of the gland and its 
connective tissue covering. v. KÖLLIKER admits, however, that other 
interpretations regarding the origin of these muscle cells are possible. 
It may be said that there is no reason for assuming that the outer 
surface of the mesoderm is always bounded by connective tissue, 
or it may be held that these muscle fibres are formed by wandering 
