ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
497 
tendency to spin nerve-processes in the course of migration. Many of 
the wandering ganglion-cells become applied to, or mixed up with, 
nerves of the transient system. Another favourite resting-place is the 
epithelium of the myotome-apex. Occasionally two occur directly oppo- 
site each other, one in each myotome of the two sides, as if pointing 
to a formerly greater symmetry. The peripheral or vagrant ganglion- 
cells are usually bipolar with two processes, but some have none. Often 
they form chains reaching from the centre to, and even over, the tip of 
the myotome. 
Transient nerves of the system . — Young embryos also show an early 
formation of nerve-processes spun by the ganglion-cells. All the pro- 
cesses, whose development is described in detail, tend either in the 
direction of the epiblast, or in that of the apex of myotome. It is 
doubtful whether the final termination of any of these nerves has been 
determined, although many were followed far below T the segmental duct. 
For a long time the nerves are merely naked axis-cylinders, and they 
are very seldom made up of more than one fibre. In later stages 
“ nuclei ” occur upon them. Most are only processes of single cells, but 
others show a plexiform arrangement of ganglion-cells and nerve-pro- 
cesses. The whole apparatus is interpreted as sensory. It reaches its 
culmination when the embryo is about 43 mm. long ; it has an unusually 
long period of functional activity, for it remains normal for two months 
or longer of the 17 months of embryonic development within the capsule ; 
degeneration begins to set in when the embryo is about 70 mm. long, 
and is complete in 12 months. 
The degeneration is described in detail. Apart from odd members 
of the vagrant ganglion-cells, the nerves appear the first to die. Their 
degeneration is followed by the slow atrophy of the ganglion-cells. The 
author also discusses the “ degradation of ganglion-cells ” — “ the losing 
of specific ganglionic characters and functions, when a cell, whose phy- 
logenetic history is ganglionic, sinks to the position of a mere nucleus 
in the conducting fibre to which it has given origin.” Permanent giant 
ganglion-cells are also discussed. In another section it is briefly shown 
that there is no morphological resemblance between the transient and 
the permanent nervous systems. 
The concluding section of the memoir treats of “development by 
substitution of organisms, or antithetic alternation of generations.” In 
the development of the skate, a sexual form (the skate embryo) arises 
by substitution on an asexual foundation or larva, the former gradually 
replacing the latter. The thesis is stated that all metazoan development 
from the egg is fundamentally an antithetic alternation of generations, 
and that directly or indirectly there is invariably a substitution of 
organisms. But this thesis, for the acceptance of which, as the author 
suggests, few are yet prepared, must be kept t apart from the embryo- 
logical and histological facts of the memoir. 
Early Development of Ganoids.* — Dr. Bashford Dean gives a result 
of his studies during the past three years on the development of Lepi- 
dosteus, Acipenser, and Amia. He thinks that their early developmental 
characters present conditions which, although in many ways dissimilar, 
* Report of the 3rd International Congress of Zoology, 1896, pp. 336-16. 
