EARLY ONTOGENETIC PHENOMENA IN MAMMALS. 7 
This outer layer has been termed by me the trophoblast, 
that inner cell-mass: the embryonic knob or “ Embry- 
onalknoten ” (88, p. 511; ’89, p. 298). E. van Beneden, while 
recognising that the trophoblast is a separate layer (’99), 
does not as yet apply that name but calls it somewhat more 
circumlocutionally the ‘“‘ couche enveloppante.”’ 
The degree of independence existing between trophoblast 
and embryonic knob is subject to considerable variation. As 
the two only become segregated when the change of the 
morula stage into a vesicle comes about, there is generally at 
the outset no such very sharp distinction, and even in later 
stages this distinction is sharper in certain genera of mammals 
(Tupaja, Figs. 21, 29; Galeopithecus, Fig. 18; Cervus, Figs. 
13, 14) than in others (Hrinaceus, Figs. 33—36; Tarsius, 
Fig. 19; Cavia, Fig. 24. Still there is reason to believe 
that in some it may be traced up to the very early stages of 
cleavage as are indicated by Figs. 1—3. The embryonic 
knob would then be represented by one or a few central 
cells, the trophoblast by the surrounding cleavage cells (as was 
already noticed above). 
When the trophoblast is being distended into a vesicle the 
proliferation of the mother-cells of the embryonic knob is 
very much slower; the total number of cells of which the 
knob is built up rarely exceeding 24—30. 
For the Opossum we have the data furnished by Selenka 
(87), to which, however, I would put another interpretation. 
The central cell of Fig. 4, looked upon by him, without 
further proof, as a hypoblast cell, is undoubtedly the mother- 
cell of the embryonic knob as a comparison with Fig. 5 makes 
all the more evident. It is, of course, important to find this 
agreement between didelphic and monodelphic mammals. 
epithelial outer layer and the inner mass to the extension of intracellular 
vacuoles (99). His interpretation has found no support in the results 
obtained by Keibel and myself, nor in those of Selenka for the Opossum. 
