The Development of the Cartilaginous Skull ete. in Necturus. 405 
The Mesectoderm in Necturus 12!/; mm long. 
The figures numbered 5 a—5e, pl. XVII, are from a series of 
horizontal sections through an embryo 12'/, mm in length. They 
represent sections through the branchial arches and show that the 
mesectoderm cells begin to group themselves more densely in definite 
regions. In the embryo of 11 mm (figs. 1 «—1:), the mesectoderm 
cells in the ventral part of the branchial arches are much more 
numerous than in the dorsal part, but show little or no tendency to 
accumulate in limited spaces within the arches they occupy. In the 
embryo of 12!/; mm, this tendency, however, becomes evident. 
Although the series of sections shown in figs. 5a—5e, are 
given for comparison with the sections of fig. 1, it is not possible 
to give planes that exactly correspond, since the head, bent down- 
wards in the younger embryo, as seen from fig. 1, begins to rise 
into the plane of the long axis of the trunk. The readjustment is 
not achieved by bending in any one transverse plane, but affects 
the relative position of all of the structures of the head. 
In the series 5a—5e, pl. XVII, I give no sections corresponding 
to the planes of figs. 1a and 14, although in the older embryo, 
mesectoderm cells have begun to group themselves near the inner, 
endodermic, wall of the branchial arches in the plane corresponding 
to that of fig. 14. It is probable that the downward migration of 
mesectoderm has been continuous, since this tissue has greatly in- 
creased in the ventral part of the branchial arches. The mesothelial 
tissue of the gill arches is still distinguished by yolk granules, 
although less conspicuously than in the younger embryo, and it is 
further found that the cells of this tissue now begin to elongate 
into muscle fibres. The change from an embryo of 11 mm to one 
of 121/, mm is not so great as to allow extensive displacement or 
replacement of tissues, and it is therefore perfeetly possible to see 
where the tissues sharply distinguished in the embryo of 11 mm 
still lie. 
I have found it impracticable to continue the diagrammatic 
manner of distinguishing the median tissues adopted in the series 
1a—1:, partly because the cells in the older embryo have become 
much smaller through repeated division, so that their representation 
on the scale adopted for clearness of distinction in figs. 1 ~—1 7, is 
even less in accordance with the actual size of the cells as compared 
with the other space relations of the section. The method is also 
