YOUNG STAGES OF LIMNOCODIUM AND GERYONIA, 199 
scheme of development of Geryonia and Limnocodium.— 
Such being the facts with regard to a limited period of the 
developmental history of Limnocodium, they can, I think, 
best be brought into harmony with the observations of 
Haeckel, Metschnikoff, and Fol on Geryonia by introducing 
between the earlier and later stages described by the two 
latter observers a hypothetical stage, as exhibited in the 
woodcuts, figs. 3, 4,5. Fig. 5 is simply a schematic re- 
presentation of the stage (three days and a half) drawn by 
Fol in his plate xxv, fig. 18, and shows the thickened 
ectoderm, which he calls “ the oral plate,” but which I 
shall call “the umbrella plate.” Fig. 5 is a schematic 
section of an eight or ten days’ embryo based upon Metsch- 
nikoff’s fig. 14 (pl. 1, ‘ Zeitsch. wiss. Zool.,’ vol. xxiv). 
Metschnikoff believes that the sub-umbral cavity is formed 
by an invagination, which at the same time gives rise to the 
oral aperture. His drawings, which are by no means 
decisive, appear to admit of the interpretation of a formation 
of this cavity by the splitting of the umbrella plate: especially 
I would refer to his fig. 11, where the enteric sac and the 
sub-umbral space are seen as two lenticular bodies closely 
applied to one another. In any case I have no doubt that 
Metschnikoff’s figures are far more nearly representative of 
the process which goes on in the formation of the sub-umbral 
space in Geryonia than are Fol’s; and the account of the 
process which he has based upon his observations, though, 
as 1 think, erroneous (owing to the absence of observations 
on embryos between the 50th and the 190th hour), are yet 
by no means so wide of the truth as those of Fol, who has 
completely failed to give even an approximately correct 
account of the matter. 
We may now interpose between the stages represented 
by figs. 3 and 5 a hypothetical representation of the stage 
not observed by Metschnikoff or Fol, basing our suggestion 
upon the fact observed with regard to the young Limno- 
codium, namely—that the sub-umbral cavity is large and 
well developed at a stage when it has no opening to the 
exterior, but is completely closed in by an imperforate pre- 
umbral lid.. Such a stage is represented in the woodcut, fig. 4. 
There is little room for doubting that the series 3, 4, 5 is true 
for Limnocodium, and that 5 and 6 are true for Geryonia. 
It will not be a difficult task to decide by the special 
examination of Geryonia embryos whether the aperture in 
the preumbral lid seen in stage 5 forms after the sub- 
umbral cavity and the oral perforation of the manubrium 
are complete, as it does in Limnocodium, or whether, as 
