No. I.] DESMOGNATHUS FUSCA. I'Jl 



in groups, passing through the medullary sheath without dimi- 

 nution of size into the field of the ventral columns. This fact 

 accounts for the diminution in size at various points in the 

 course of the fibers. 



" The shape of the multiaxial fibers varies according to the 

 part of the spinal cord in which they are observed ; at the pos- 

 terior end they are elliptical, towards the center they are round, 

 and further forward they are almond-shaped. 



"At a short distance posterior to the point where the facial 

 emerges from the brain, this axis-cylinder is the only one 

 remaining, all the others having disappeared, the whole fiber 

 having in the meantime diminished in size. The fiber has now 

 quite the appearance of the Mauthner's fiber of the Teleostei, 

 consisting as it does at this point of a single large axis-cylinder 

 surrounded by a thick medullary sheath. Immediately in front 

 of this spot the axis-cylinder of one side decussates with that 

 of the other, the place corresponding with the position of the 

 decussation of the Mauthner's fibers in the Teleostei." 



Summary. 



1. The preservation of so many embryological features both 

 in the morphology and histology of the neuraxis of the Des- 

 tnognathiis is quite remarkable. Throughout the whole length 

 of the tube there may be recognized the homology of the three 

 layers as described by His. In the early embryo the ental layer 

 is composed of epithelium or endyma; the middle or mantle 

 layer consists of the cinerea, and the ectal layer of alba. The 

 nerve cells appear to be scarcely more than fairly well developed 

 neuroblasts. The regeneration of lost parts in the Urodeles is 

 well known, and this phenomenon may be more or less depend- 

 ent upon the embryonic condition of the elements of the neur- 

 axis and their power of rapid growth utilized in this direction. 



2. The cephalo-caudal compression of the brain parts is 

 quite marked and undoubtedly exerts a strong influence upon 

 the peculiar relations of the seventh and eighth nerves. 



3. The crista in the Dcsmogtiathiis is nothing more than a 

 membranous intrusion of pia into the aula. 



