54 



to pass through the body of one mesectoderm cell. So large are the 



cells and nuclei in Necturus. 



Fig. 3 is a camera drawing of two cells from a section through 



the anterior region of the head in an embryo at a stage of develop- 

 ment about midway between the stages 

 ^ represented in figs. 1 and 2. a is a 



typical representative of the wandering 

 mesectoderm cell at this stage, and b 

 of a wandering mesendoderm cell. It 

 is evident that a stain which is taken 

 by the yolk globules would differentiate 



Pig. 3. a mesectoderm cell, 6 mesendoderm 

 cell, n nucleus, yk yolk globules. 



such cells effectually and destroy the possibility of confounding them. 

 (For this purpose the triple stain known as "Biondi-Ehelich" gives 

 most satisfactory results.) 



It may be thought that at some intermediate stage the yolk gra- 

 nules in the mesendoderm cells become reduced, and that these cells 

 then add themselves indistinguishably to cells migrating from the 

 neural crest. Such, however, is not the case, for my series of recon- 

 structions show that the line of (demarcation between mesectoderm 

 and mesendoderm is always sharp. These stages, moreover, are not 

 rapidly passed as in the bird, but prolonged over a week or two — 

 the time varying with the temperature. 



In their migration, the mesectoderm cells lie between the ectoderm 

 and mesendoderm, but when they reach the point where the mesendo- 

 derm is parted to admit of the fusion between ectoderm and endo- 

 derm at the gill clefts, they become somewhat more closely grouped 

 and also receive here extensive ectodermic additions (Beard's branchial 

 sense organs) after which, part of the migrating cells are permanently 

 arrested and form with the above mentioned ectodermic proliferations 

 the Anlage of the cranial ganglia; the remainder continue their downward 

 path, and migrating over the edge of the mesendoderm where it is inter- 

 rupted by the forming gill cleft, these wandering mesectoderm cells 

 come to lie also on the inner (endodermic) side of the branchial arches, 

 between the endoderm and mesendoderm. Their position there is in- 

 dicated by crosses in fig. 2. 



It will be seen that my account of the origin and fate of the 

 neural crest in Necturus differs from the accounts usually given 

 for other vertebrate embryos chiefly because the yolk differentiation 



