181 
as that of Polypterus, i. e. with ventral origin, within the region of 
the gill-clefts+). 
Indeed, I take it that such a mode of origin of those peculiarities 
would be more reasonable than the supposition that they were due to 
a new formation’). 
The tendency of one school of research, and of that I claim to be 
a member, is to carry the origin of organs from other organs as far 
back as possible. 
The Harveian dictum might be extended for us as omne orga- 
num ex organo. We look with suspicion on supposed new forma- 
tions, until we can shew that they are derivable in an intelligible way 
from preexisting organs. In my humble opinion KLEINENBERG’S essay 
„Über die Entwickelung durch Substitution von Organen“) is the 
finest chapter of embryology that has ever been written. The consider- 
ation of its principles furnishes the key to the comprehension not only, 
as KLEINENBERG demonstrated, of the manner in which the Annelid 
arises, but also of that in which the Vertebrate develops. 
When I consider the branchial apparatus of Petromyzon as a 
compound of gill-clefts and swim- bladder I am hardly assuming a 
change of function, only the fusion of one organ with another set of 
organs. 
There are, I need hardly say, many parallels of such a case. Into 
the service of the jaws skin-scales enter, and become teeth: into that 
of the lateral sense organs the segmental mucous sacs‘). 
1) This is only suggested as a possibility. There are Teleostei in 
which no swim-bladder is met with. Like the limbs it may have com- 
pletely degenerated in the Marsipobranchii. The apparent absence of a 
swim-bladder in these forms does not weaken the other evidence of their 
alliance to the Ganoidei, otherwise one must logically conclude the non- 
relationships to one another of all those Teleostei with swim-bladder 
on the one hand, and those without it on the other. 
2) The metamorphosis of Petromyzon, a matter unfortunately hitherto 
much neglected, is undoubtedly a reversion to an ancestral condition, as 
Donen has so often insisted. This is seen from many circumstances, the 
development of the eyes, of the parietal eye etc. The Ammocoetes — a 
larva modified for a special mode of life — becomes more fish-like. 
The absence of a swimming bladder in the larva is hardly a difficulty, 
for that form has only retained and developed such organs as it abso- 
lutely needs. 
3) Zeitschrift f. wiss. Zool. Bd. XLIV, 1886, p. 212. 
4) How close this latter connection has become shall be made clear 
subsequently. I hope to show reasons for maintaining not only that 
