338 The American Naturalist. 
Miss Platt (’89) finds that in the chick the first mesodermal 
cleft occurs anterior to the first protovertebra, and that two proć 
tovertebræ are subsequently formed anterior to this cleft. The ; 
four pairs of protovertebræ entering into the formation of the 
„head are thus evenly divided by the first mesodermal cleft. Dur- 
ing the second and third days of incubation the medullary tube 
becomes divided by a series of constrictions into vescicles or 
neuromeres. Anterior to the first protovertebra there are seven 
of these neuromeres. As the protovertebre are successively 
formed, neuromeres are added, each opposite a protovertebra; but 
of the latter. The anterior neuromere gives rise to the prosen- 
cephalon, thalamencephalon, and mesencephalon. The develop- 
ment of these three brain vescicles is coincident with the cranial 
flexure, and the latter may be due to the rapid development of 
the dorsal and lateral walls of the first cerebral vescicle. From 
the second neuromere is developed the cerebellum. The succeed- 
ing vescicles, including those between the first five protovertebra, : 
these medullary folds observed by them in the lizard and chick - 
are the same as those observed by Platt in the salmon and c 
The, primitive relation in the chick is different. The V. nerve 
arises not as Béreneck says, from the outward convexity of the a 
neuromere of the medulla, but from the concavity between 7 
first and second neuromeres. Opposite this concavity a Tia; 
projects into the fourth ventricle, ‘composed of lines of cells í 
verging like the rays of a fan toward the point of origin of the 
nerve. At the time when the VII. and VIII. nerves have just i 
the neural ridge, from the concavities between the second 
third and the third and fourth neuromeres spring nerve 
which unite in a large ganglion. Thus at an early per 
VII. and VIII. nerves are distinct from each other, but as the ti 
neuromere is smaller than the others the space between the 
5 See Orr. Journ. Morph., Vol. I., No. 2, p. 335. 
4 
