SOME TEACHINGS OF DEVELOPMENT. 
207 
Still, even in such cases as these the cup-stage is not absent. 
The same result is obtained, but in a different way. Tor the 
enclosure of the pabulum-containing entoderm by the ectoderm 
is effected solely by the growth of the latter, without any accom- 
panying invagination of entoderm. In these cases the cavity of 
the gastrula also is necessarily occupied by the pabulum, and 
this if greatly in excess may even project beyond the cup-orifice. 
We may then take the gastrula or cup-phase as a second stage 
and note that, like the first, it is met with (modified only by the 
accidental presence of food material) in all animals above the 
Protozoa. 
There are two apparent exceptions to the general rule of for- 
mation of the gastrula stage by invagination. These are met 
with in the development of the freshwater polyp (Hydra) as 
described by Kleinenberg, and that of one of the Medusse (Ger- 
yonia) as described by Metschnikoff and by Pol. In these 
animals the bilaminar blastoderm is said to be formed by the 
splitting into two portions, inner and outer, of some or all of the 
cells of the single layer which forms the wall of the blastosphere, 
the inner parts becoming collectively the entoderm, the outer 
remaining as ectoderm. Eegarding these exceptions the ques- 
tion naturally suggests itself. Are they due to defects of obser- 
vation ? and further, supposing that there is’no flaw in the facts, 
are they nevertheless explicable as modifications only in the 
ordinary mode of formation of the gastrula ? Certainly as far as 
concerns Hydra, the recorded observations of Kleinenberg are 
too incomplete on this part of the development for his conclu- 
sions to be accepted without demur. And it is very generally 
admitted that the development of Geryonia needs careful rein- 
vestigation with the aid of microscopic sections. It is not 
improbable that the result of such reinvestigation might prove 
the existence of some sort of invagination at a much earlier 
period than that at which the so-called delamination or splitting 
occurs. 
The cavity of the cup at first communicates, as we have seen, 
with the exterior, by means of the aperture of invagination or 
protostome, and when this becomes closed, or even before its 
closure, another aperture appears, and in most cases yet another. 
These secondary openings into the cavity of the gastrula become 
respectively the anterior and posterior orifices of the alimentary 
canal, and their formation is always accompanied by an ingrowth 
of ectoderm towards the endoderm. One or other of these aper- 
tures may be formed in the place which was before occupied by 
the now obliterated protostome, or in some cases the latter may 
itself remain persistent as one of the secondary orifices. 
\\ e may now pass to the consideration of the third essential 
