373 
remained so, until some little time after the transient apparatus of 
Acanthias had been found. 
Here is the problem in its proper form. 
In Raja or Scyllium, in which the development takes place 
in the sea, there is a considerable transient nervous apparatus of 600 
ganglion-cells or more. In three forms, in which the development is 
uterine, a gradual disappearance of the transient apparatus can be 
established, such that, while in Acanthias some 50 ganglion-cells 
may be developed, in Mustelus vulgaris this number is reduced 
to 12 or less, and in Torpedo not a single one has, in spite of much 
search, been lighted upon. How is the gradual disappearance to be 
accounted for? How does it happen, that the initiation of uterine de- 
velopment will apparently only suffice as an explanation of the ab- 
sence of the cells in Torpedo, and not as a reason for the presence 
of a few of them in Acanthias and Mustelus? There must be 
something at the bottom of the apparent anomaly; some natural ex- 
planation must exist. 
It is, indeed, a very hard nut to crack, as anyone will find, if, 
without seeing what follows, he should attempt to seek the key. 
The solution attempted is offered for what it is worth. It had 
to be solved in some way or other, and the solution sought after 
had to be prima facie of a reasonable kind. After much thought 
the idea occurred that the presence or absence of an egg-shell or 
purse might be at the bottom of it all, for it was recollected, that 
there is stated to be no egg-shell formed in Torpedo, and that 
one has been more than once described as present in Acan- 
thias. 
A maxim for the Vertebrate morphologist might be “when in doubt 
as to questions of fish-morphology consult JOHANNES MULLER” !). 
When this stage had been reached, it became, indeed, only a 
question of once more reading one of his classic works to find a so- 
lution, which, in the absence of any other, must be the solution. 
MÜLLER on p. 235 quotes from Home as to the existence of an 
egg-case in Acanthias vulgaris, and speaks himself of finding 
the remains “eines Balges”. There is, indeed, a fairly well-developed 
ege-case in this form. 
1) Jonannes Mitten, Ueber den glatten Hai des Aristoteles, und 
über die Verschiedenheiten unter den Haifischen und Rochen in der 
Entwickelung des Eies. Abhandl. d. K. Akad. d. Wiss. Berlin, 1840, 
ip. 187-957. ar) 
