392 Julia B. Platt 
The mesectoderm forms in the trigeminal region, a wide sheet 
of tissue extending from the neuropore to the posterior boundary of 
the mid-brain, but the invagination of the auditory epithelium soon 
separates the facial mesectoderm from that in the region of the 
glossopharyngeus and vagus Anlagen. From their neural, dorso- 
lateral, and epibranchial lines of origin, the mesectoderm cells wander 
downwards immediately below the superficial ectoderm. On reaching 
the places where the ectoderm has fused with the endoderm to form 
the mouth or gill clefts, the mesectoderm, continuing its downward 
migration, turns inwards between the mesoderm and the endoderm, 
and thus finally comes entirely to surround the mesoderm of the 
branchial arches and to underlie the invaginated ectoderm on the 
anterior and posterior (ultimately dorsal and ventral) surfaces of 
the mouth. 
Meanwhile the dorsal cells of the mesectoderm group themselves 
into the cranial ganglia, and become distinct from the connective 
tissue of which the ventral part of the mesectoderm is composed. 
This mesectodermic connective tissue receives further additions from 
the ectoderm in the neighborhood of the mouth and in the branchial 
arches. 
Referring to my former paper (loc. cit.) for a more detailed 
account of the origin of the mesectoderm and its relation to the 
mesoderm in the head of Necturus, I now pass to the description 
of the stage of development with which this study begins. 
b) Descriptive. 
The Mesectoderm in Necturus 11 mm long. 
My ealier study (94) closed with the embryo 10 mm in length, 
and I begin this study with a stage of development immediately 
following, giving in fig. 1, pl. XVI, the head of an embryo 11 mm 
long. Figs. 1a—17, show horizontal sections through planes indicated 
in fig. 1 by lines lettered to correspond with the letters below the 
respective sections figured. Throughout this series of sections, the 
several kinds of tissue are diagrammatically represented. The out- 
lines of the tissues, however, were traced with the camera, and their 
relations are consequently accurate. The cells of the mesectoderm 
are represented in a solid shade, while the mesoderm cells are 
represented by circles. The cellular structure of the ectoderm and 
endoderm is not reproduced, as the series is given to show the 
