242 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



divisions of encephalomere III, as may easily be determined by a com- 

 parison of his figures with frontal sections of swine embryos. He 

 says (p. 188), " Die ventrale Ausbuchtung der zweiten Falte kann man 

 also mit voUkommene Sicherheit fiir die beginnende Ponsanlace, und 

 ihren oberen Theil fiir die Cerebellumanlage halten." But, as may be 

 determined by the relations of the neuromeres to the ear capsule and to 

 the ganglia of the acustico-facialis and the trigeminus in his figures 

 (Taf. X. Figs. 4, 5), the true Anlage of the cerebellum lies anterior to 

 this and is his " erste Falte," which on theoretical grounds he considers 

 related to the trochlearis nerve. As a result of this mistake it happens 

 that tlie neuromere which he calls " Abducensneuromer " (VI), and to 

 which on purely theoretical grounds he assigns the sixth nerve, is in 

 reality encephalomere V, which is connected with the acustico-facialis. 

 With this neuromere the ahducens never is connected in any Vertebrate yet 

 studied. In the swine, as I am able to afiirm from my own observations, 

 the abducens arises from the ventral portion of hindbrain neuromere VI, 

 which in the early stages of all Vertebrates lies opposite the ear capsule. 

 In Xecturus, the chick, and S. acanthias, its origin is ventral and poste- 

 rior to the origin of the acustico-facialis. In support of this theory of 

 the mechanical origin of the " Falten," Broman finds that, as a result, 

 as he thinks, of the flexure of the neural tube, those neuromeres which 

 correspond with encephalomeres IV and VI of my figures are wedge- 

 shaped, and that their ventral edges do not reach the ventral wall of the 

 neural tube. Moreover, none of his neuromeres extend to the " Deck- 

 platte." But a study of swine embryos leads me to conclude that this 

 is not characteristic of all mammalian embryos, and indeed that it may 

 be "ein blosser Zufall" in the case of Broman's human embryo. In 

 young swine embryos (killed 19 days after coitus) none of the neuromeres 

 are wedge-shaped ventrally or dorsally ; moreover, the constrictions be- 

 tween them extend into the Deckplatte. The posterior constriction 

 of encephalomere V extends across the Deckplntte until a somewhat 

 later stage, and in this constriction a mass of neural-crest cells persists 

 in a way precisely similar to that in which neural-crest cells in S. acan- 

 thias persist in the regions of constriction between the primary brain 

 vesicles (encephalomeres).^ 



From an examination of the evidence presented by those who have 

 held that the neuromeres are purely the result of mechanical influences, 



1 In connection with this fact, it is to be noted that the walls of the medulla in 

 this region are little distended laterally, which may be ascribed to the influence of 

 the ear capsule. (See Plate 5, Fig. 30.) 



