374 
Regarding Mustelus vulgaris!) MÜLLER’s account is much 
fuller, for on p. 217 he writes: “Das Ei von Mustelus besteht im 
Uterus vor der Entwickelung des Jungen aus Eischale, Eiweiß und 
Dotter. Die Eischale ist ein äußerst feines horniges Häutchen von 
der Dünne des Amnions der höheren Tiere?), diese gelb- 
liche, völlig durchscheinende Haut ist ganz structurlos ete.” On p. 239 
he adds concerning the remaining form: — “Die Eier von Torpedo 
besitzen nichts von einer Schalenhaut etc. etc.” 
It appears reasonable to maintain that the development, though in- 
fluenced by the changed conditions, does not become, strictly speaking, 
completely uterine, until the egg-case entirely ceases to be formed. 
If this be admitted, we arrive at the following conclusions. 
In the case of Raja or Scyllium the original free larval de- 
velopment of the phorozoon or larva has, as in Asterina gibbosa 
among Echinoderms, been altered to a modified and restricted larval 
life within an egg-case formed for protection. In Acanthias the 
modification has gone still further, and the egg-case in its turn is 
protected within the maternal uterus; but a fairly developed egg-case 
is formed, and thus the larval nervous system has a fair justification 
for development, albeit in reduced form. In Mustelus vulgaris 
the egg-case is reduced to an excessive thinness, and correspondingly 
the transient apparatus is diminished almost to nothingness?), and, 
finally, in Torpedo no egg-case whatever is formed, development is 
completely uterine, and the final traces of the transient nervous system, 
— not of the larva or phorozoon! — have disappeared *). 
A different and more natural explanation may occur to some other 
embryologist. If one such be possible, it would interest me exceedingly 
to hear of it. 
1) MÜLLER mentions no distinction in the egg-shells of the two 
species of Mustelus. 
2) The spacing is mine. 
3) The rate of increase or diminution will probably always be in 
geometrical progression, because it is determined by the existence or sup- 
pression of cell-divisions. | 
4) It would be interesting to learn at what phase of the develop- 
ment the embryos of Acanthias and Mustelus leave the egg-case, it will 
probably turn out to be at the critical period. 
