58 OSBORN. [Vor. II. 
ures). There is thus a striking similarity in the fibre courses 
of these three dorsally decussating tracts. In Cryptobranchus, 
in which we have the most primitive type of brain thus far 
observed among the Amphibia (Plate IV., Fig. 7), these three 
tracts are nearly subequal, the superior commissure containing 
the largest proportion of fibres. 
This anatomical evidence for the serial homology of these 
commissures is supported by the facts of their embryological 
development. I have followed these stages in the frog, in which 
the superior commissure is extremely reduced ; it would probably 
be much more clearly shown in C7yptobranchus embryos. These 
commissures develop nearly, if not quite, simultaneously with 
the anterior commissure, at the period immediately following the 
constriction of the neural tube into four vesicles. In the dorsal 
median line this constriction is clearer between the two posterior 
segments than between the first and second (compare co/ and 
pem in Cut 1 with scm).4 But in horizontal sections these three 
constrictions are equally great, Cut 2. It is noteworthy that 
the floor of the neural tube, which evidently has no relation to 
these dorsal commissures, is also the only region in which from 
the first there is no constriction between these vesicles, the 
constrictions occurring, first, at the point of the cranial flexure ; 
second, opposite the anterior commissure, acm. 
The inference to be drawn from these facts depends largely 
upon the question whether there is really a serial homology be- 
tween these commissures in their primitive condition. If not, 
there remains considerable ground for the supposition that the 
intersegmental folds are lines of retarded growth in the sides 
and roof of the neural tube to be traversed at an early period by 
the commissures. 
In the last number of this Journal, Orr (87, p. 347) has 
contributed valuable observations on the lizard’s brain, which 
certainly support, and possibly extend, this hypothesis to em- 
brace also the anterior commissure. He finds, what I had not 
observed, that the fibres of these commissures are possibly con- 
tinuations of the primitive lateral longitudinal fibres, although 
not positively observed to enter them, concluding as follows: 
“The superficial position of these three commissures, anterior, 
superior, and posterior, their similar connections with the lateral 
1See Appendix. Note 1. 
