336 
task of suppressing the larval foundation or asexual form upon which 
it had arisen. | 
It may be worth recording how the recognition of this stage came 
about. In the long course of the work, and not by any means near 
its commencement, there arose the idea of making notes of the con- 
dition of various organs in different embryos with a view to future. 
work on some or other of them. In doing this there was not the 
slightest conception at the time that these notes would be of use for 
the work in hand. It was only long afterwards that the importance 
of the changes wrought at a certain period became apparent. 
Ever since then the importance of this critical period has seemed 
to increase, and no pains have been spared to analyse it more fully. 
For the moment there is no intention of writing an essay‘) on the cri- 
tical period. My concern, at this juncture, is to relate certain other 
— perhaps to some, trivial — things which then happen. 
In once more taking up research on Lepidosteus and Scyl- 
lium the first inquiry which presented itself was that as to the pres- 
ence or otherwise of a critical period in these forms. With the find- 
ing of this in Scyllium a resolve was made to carry out a very com- 
plete examination of the development at, and on each side of, the 
period in this form. This was all the easier, because the embryos of 
S. canicula of these stages were far more abundant than, and barely 
half as large as, those of Raja batis. 
Bd. 44, p. 116) KLEINEnBERG writes: ‘Die Rückbildungen des larvalen 
Nervensystems (beginnen), wenn die Anlagen der Organe des Annelids eine 
gewisse Selbständigkeit erreicht haben.” This, like many other passages 
in his work, is very similar to expressions, which have been used by myself 
in my recent memoir. And naturally so; for the line of work was so 
similar in the two cases, although the conclusions were in important re- 
spects very different. During the actual writing of my own work occasion 
only once arose to refer to KLEINENBERE’s memoir, — in order to copy the 
exact title of Cap. VI — and the similarity of expression was only brought 
about by a similar line of thought. Recently it occurred to me, that it 
would be important to know if Kırınznsere had defined more precisely 
the period, at which the involution of the larva began, but beyond this 
passage nothing could be found. As in a great many other cases I con- 
clude that there must exist a critical period in the Lopadorhynchus de- 
velopment, — it may correspond to Kremenseres Fig. 13 Taf. I, or 
to a stage a little earlier than this. 
1) In the meantime such an attempt has been published elsewhere. 
Vide “On certain Problems of Vertebrate Embryology”. Jena, Verlag von 
G. Fischer, 1896. 
