In the formulation of a scientific arrangement of the Ichthy- 
opsida the facts of comparative embryology, as well as those 
of comparative anatomy, must be taken into account. About the palae- 
ontological aspect of the matter I do not care to say much — perhaps, 
later on, an opportunity may be found for its further consideration. 
In May 1889 I published a preliminary note on the development 
of Lepidosteus +). 
Some of the facts recorded appeared to have bearings on wider 
morphological questions. Acquaintance with the work of others on Se- 
lachii, Ganoidei, Marsipobranchii and Dipnoi: actual work, 
partly for my own information, on the comparative anatomy and, as 
far as practicable, on the development of these groups, afforded an 
opportunity of forming an estimate of some of the broader rela- 
tionships of the forms constituting the Ichthyopsida. 
It was intended that the subject of this paper should be incor- 
porated in a memoir on Lepidosteus. The time necessary for the 
completion of that memoir, and the delay arising out of the 
need of more material, especially for purposes of comparison, lead 
me to offer this preliminary paper. Criticism is desired, for mistakes 
are made by everybody, but that criticism which begins and ends in 
Amphioxus as the originator of all that is good (and bad) in the 
Vertebrata may be spared, unless it be also demonstrated how the 
Vertebrata got their various organs from that animal. 
Whenever Amphioxus is placed in connection — no matter how 
remote — with existing Ichthyopsida, whether as ancestor or as de- 
generate fish, its claims are based on false and visionary pretences. 
If we take the ancestral tree of the fishes, according to HAECKEL 
or RABL, one is struck by two facts. Firstly, some of those forms 
with large eggs, e. g. Selachii, produce a much smaller number of 
eggs, than those whose eggs contain but little food-yolk, e. g. Ganoi- 
dei and Petromyzon. 
The explanation, which, moreover, is an admission not without 
significance, as will be seen in the sequel, is very simple according to 
a morphologist of the school of Prof. Rast. As much energy is em- 
ployed in the production of one large egg as in the ripening of a 
large number of small ones. 
The second fact, for which we have the same hypothesis used 
half a dozen times over by way of explanation, is that the eggs of 
1) Proc. Roy. Soc. Lond., May 16 1889. 
