468 'Journal of Comparative Neurology and Psychology. 



One new idea is introduced into the subject of head morphology, 

 namely that in the mesoderm the two structures in each segment 

 heretofore known as the somite and the branchial arch together con- 

 stitute the somite, and the junction of the branchial arch with the 

 pericardial cavity is to be compared with the junction of the trunk 

 somite with the peritoneal cavity. This conception is used in support 

 of the view that mesomerism and branchiomerism coincide. It may 

 be questioned whether the new conception is not more in need of sup- 

 port than that for whose support it was called in. The material 

 which these authors have studied is far from adequate for the study 

 of the primitive seg-mentation of the head or the morphology of the 

 cranial ner\'es. This is especially noticeable in their statements re- 

 garding the anterior head cavity, which could not be studied in the 

 material on which they worked, and the other head somites, whose 

 development is well advanced in the earliest stage represented in their 

 material. In determining the first segment of the head no value is 

 attached to the first two brain vesicles, the eye, the olfactory nerve, 

 the nervus terminalis, the rudimentary nervus thalamicus, or the 

 epiphyses. The first segment is that of the premandibular somite 

 and the ophthalmicus profundus (to which the name Ciliarganglion 

 is given). This nerve is related to the brain behind the cerebellum 

 in the embryos studied by Ziegler and his colleagues, but whether 

 they would include all the brain in front of the cerebellum in the 

 first or premandibular segment is not stated. Most of the processes 

 upon which an intelligent judgment regarding the primitive segmenta- 

 tion of the head can be based have been completed in selachian em- 

 bryos prior to the earliest stage studied by these authors. 



With regard to the mesectoderm derived from the neural crest 

 which extends down into the mandibular and other visceral arches 

 from the cranial ganglia, I can fully confirm the statements made by 

 ISTeal and illustrated in his Plate 3 (1898). To these statements 

 one addition requires to be made which is at the same time an addi- 

 tion to the early history of the anterior head cavity and preoral ento- 

 derm. The details of the earliest appearance of the anterior head 

 cavity have not been given by Miss Piatt or by ISTeal, who was con- 

 cerned chiefly with neural segmentation, and Dohrn's (1904) treat- 

 ment of the anterior head cavity is unsatisfactory. 



