212 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



up to the time when a tihrillar connection of the nerves with the neural 

 tube is effected and the chief peripheral branches are differentiated.^ 



Minot ('92) and Mitrophanow ('93) have stated that the neural crest 

 in Selachii is not differentiated before the closure of the neural tube, and 

 Eabl ('89) found that in Pristiurus embryos the " Trigeminus Anlage " 

 first appears at a stage with 18 somites. On the other hand, Beard 

 ('88) and Dohrn ('90) have shown that in some Selachii,^ as well as in 

 Sauropsida, the neural ci'est is differentiated in the region of the head 

 before the closure of the neural tube. 



As has been previously stated, my observations confirm those of Beard 

 and Dohrn, since I find that at an early stage, when the cephalic plate 

 is still widely open, the fundament of the trigeminus is clearly differen- 

 tiated from that portion of the neural plate which is destined to form 

 the neural tube. The disassociation of the neural-crest cells in this 

 region and their resultant loss of compact arrangement have taken place 

 to a considerable extent before the neural folds meet in the mid-dorsal 

 line. Usually the neural folds first close in the trunk region behind the 

 cephalic plate, and later in the region of the midbrain, i. e. in the region 

 of the " Trigeminus Anlage." The closure of the cephalic plate occurs 

 last in the forebrain, where the " neuropore " persists for a considerable 

 period. 



At a stage with 15 or 16 somites (Plate 3, Fig. 7), when the cephalic 

 plate is closed except in the region of the forebrain, the neural crest is 

 clearly differentiated in that region of the brain which extends from the 

 constriction between forebrain and midbrain to the anterior constric- 

 tion of hindbrain neuromere (encephalomere) IV, i, e. in the region 

 of the so called cephalic flexure. In the region of encephalomere IV 

 a few cells with protoplasmic processes occur in the space between the 

 neural tube and the overlying ectoderm. These may indicate that at 

 one time this encephalomere was a region of cell proliferation and thus 

 possessed a neural crest ; but since the cells soon disappear, and since 

 no new ones take their place, this encephalomere may be said to be a 

 region of the neural tube which now (in S. acanthias) possesses no neu- 

 ral crest. That portion of the neural crest which arises anterior to this 

 ueui'oraere has been variously called " Trigeminus Anlage," " germe du 



1 A study of the liistogenesis of nerve has been made only in the case of the 

 eye-muscle nerves, whose morphology still remains a matter of much dispute. 



2 I am surprised by Hoffmann's ('94) statement that in S. acanthias the tri- 

 geminus Anlage first appears in an embryo with 17 somites, that is, after the closure 

 of the neural tube. 



