212 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
up to the time when a fibrillar 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 
Rabl ('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 crest 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 enéephalomere 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 
neuromere has been variously called “ Trigeminus Anlage,” “ germe du 
1 A study of the histogenesis 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. 
