The Development of the Cartilaginous Skull etc. in Necturus. 387 
leave the neural crest and the external ectoderm in Necturus, do 
not form with wandering cells from the mesoderm, a homogeneous 
tissue, but remain for some time distinct. 
An account of the origin of these cells, which I group together 
under the term mesectoderm, has been given in my first study 
on the ontogenetic differentiations of the ectoderm in Necturus 
(94), and is reviewed at the beginning of the second division of the 
present paper under the heading Procartilage. In this first study, 
I followed. the migrations of the cells of the mesectoderm until the 
stage of development in which these cells divide into two groups, 
one composed of nervous tissues, and the other of connective tissues. 
A second study (96) was concerned with the development of the 
peripheral nervous system, while the present study discusses the 
further fate of that part of the mesectoderm which remains as 
connective tissue. 
PartI and Part II of this study are separated by the appearance 
of true cartilage. In Part III those facts regarding the development 
of the branchial muscles, which have been noted in the first part 
of the study, in association with the development of the procartilage 
of the branchial arches, are brought together and extended, and the 
origin of the hypoglossal musculature is described, with a short 
discussion of the occipital segmentation. A few points of interest 
which appear in the second and third parts of the study, are noted 
in a summary at the end. The conclusions reached in the first part 
of the study — that concerned chiefly with the formation of pro- 
cattilage — are discussed below, in the introduction, because of 
their direct bearing on the theoretical and actual value of the middle 
germ-layer. 
I find that the branchial cartilages and the anterior part of the 
trabecular bars are formed in Necturus from mesectoderm, i. e. 
wandering cells of ectodermie origin. The posterior part of the 
trabecular bars, the basal plate of the chondrocranium, the auditory 
capsules, and the occipital arch are formed in mesenchyma of purely 
mesodermic origin. The mesectoderm appears to take no part in 
the formation of the endothelium of the blood vessels, nor does the 
mesectoderm take part in the formation of muscular tissue. It 
composes, however, the connective tissue which underlies the epi- 
thelium of the mouth at the time when the teeth first appear, and 
is consequently the source of the primitive dentine, not by direct 
proliferation from the epithelium of the mouth, as maintained by 
26* 
