EARLY ONTOGENETIC PHENOMENA IN MAMMALS. 63 
value of a lecithophore, is an unwarranted hypothesis in the 
same direction as that of the gastrulation in two phases, of 
which Keibel and myself were the godfathers many years ago 
(Keibel [’89], Hubrecht [’88]), but which we have both 
abandoned since. 
IV. FisHes. 
The early developmental phases of the germinal layers of 
the fishes have not been the object of personal observations 
of any extension on my own part, although I possess a certain 
number of section series both of Klasmobranchs and Teleosts. 
And so I only intend, in the following pages, to give some 
eleanings from the literature on the subject, which appear to 
me to point to the possibility that the views to which in- 
vestigations of the mammalia have led me may algo hold good 
for these lower vertebrates. 
Beginning with Amphioxus, I need only allude to Legros’ 
latest contribution to the ontogeny of this animal, from which 
I have copied Fig. 120. In it we see the region marked pp, 
singled out by Legros as a part of the original entoderm 
of the wide-mouthed gastrula, which in Amphioxus is formed 
—in contradistinction to all other Vertebrates—by invagina- 
tion, and not by delamination (cf. p. 13). 
Legros (07) and Cerfontaine (’06) are willing to adopt the 
essential points of Lwoff’s interpretation, which has been so 
fruitful in setting other observers to pause and reftect. The 
part marked pw in the longitudinal section (Fig. 120) of a 
stage of early notogenesis should thus be looked upon as the 
median portion of what is Lwoff’s “ Dorsalplatte,” and as an 
essentially ectodermal derivate which has come about in con- 
sequence of the growth backwards of what was originally the 
dorsal lip of the early blastopore (cf. for mammals with Figs. 
48,49, and 97—99 of 'Tarsius). During this backward growth, 
which does not, as is the general belief, complete gastrulation 
(see Hubrecht, ’05), but which initiates notogenesis, the two 
embryonic regions become distinguishable, which, also in 
