52 



common type, often neglect sufficiently to note existing differences. 

 As there are some seemingly important differences between the de- 

 velopment of the neural crest in Necturus and that described by 

 Goronowitsch for the bird, I venture to submit this further pre- 

 liminary notice of a paper which I hope soon to publish. 



The term "mesenchyme", which is used by Goronowitsch (loc. 

 cit.) to include without distinction cells migrating from the neural 

 crest and from the axial mesoderm, is evidently inadequate to a de- 

 scription of the condition found in Necturus, where the cells so 

 migrating do not form, as in the bird, a more or less homogeneous 

 tissue, but preserve their distinguishing differentiation to a stage in 

 the development of the embryo later than that of the secondary con- 

 nection of the cranial ganglion with the brain. For practical con- 

 venience, therefore, in my description of Necturus, I shall divide 

 the cells which occupy a position between the two primitive germ 

 layers into two groups, mesectoderm and mesendoderm 1 ), 

 according as they are respectively derived from one or other of the 

 two primitive layers. Mesendoderm thus includes somites, parietal 

 and splanchnic "mesoderm". 



As has been hitherto stated for many vertebrates, the neural 

 crest of Necturus arises in two divisions, the anterior of which lies 

 above the first and second primary cerebral vesicles. The posterior 

 division begins above the third primary vesicle, just anterior to the 

 Anlage of the auditory epithelium and extends backwards to the 

 posterior extremity of the body. The continuity of the neural crest 

 in Necturus is therefore interrupted but once, while, as Goro- 

 nowitsch tells us, in the bird a second interruption occurs above 

 the auditory vesicle. 



The diagrams (figs. 1 and 2) here given are reduced reconstructions 

 from camera outlines. The line (ect.) where the ectoderm passes off 

 over the yolk, as well as the flexure in the floor of the brain indicate 

 the relative stage of development. 



Fig. 1 shows the position of the neural crest in the head of Nec- 

 turus when the cells have begun to migrate from their original position 

 above the brain, but are still in continuity with its mid-dorsal wall. Bet- 



1) I am aware that these terms have been already employed by 

 Kleinenbebg and others to designate indifferent tissues from which on 

 the one hand mesoderm and ectoderm, on the other mesoderm and. endo- 

 derm are derived, but trust that the liberty I take in using the words 

 for another purpose is warranted by their exact interpretation of actual 

 conditions in Necturus. 



