DIFFERENTIATIONS OF ECTODERM IN NECTURUS. 485 
Ontogenetic Differentiations of the Ectoderm in 
Necturus. 
Study II—0On the Development of the Peripheral Nervous 
System. 
By 
Julia B. Platt. 
a 
With Plates 36—38. 
INTRODUCTION. 
IT unpErTooK the study of the development of the lateral 
line system in Necturus at the suggestion of Professor C. O. 
Whitman, and am indebted to Professor K. Brooks for access 
to the literature of my subject. As the questions of chief 
interest to me are connected with the relation of this system 
to vertebrate segmentation, and to the formation and distribu- 
tion of the cranial nerves, I turned to the earlier stages of 
embryonic development to discover there, if possible, the first 
indications of ectodermic differentiation which might serve as 
the source either of cranial ganglia or of sensory specialisations 
in the skin. This led to the discovery that large numbers of 
cells, arising in the ectoderm and migrating below the surface 
of the body, take part neither in the formation of ganglia nor 
nerves. They are, however, distinguished in Necturus from 
the surrounding tissues by marked difference in the size of 
the yolk granules they contain. 
I grouped together the ganglionic and connective-tissue 
cells which thus migrate inwards from the ectoderm under a 
common term, “ mesectoderm,” and in a former paper (82) 
