ON THE INVAGINATE PLANULA. 163 
digestive cavity, as is well known to be the case in all the 
Hydrozoa, and in the higher Metazoa which have been spe- 
cially examined on this point. Whilst there is room for 
much doubt in favour of the anus sometimes coinciding with 
the orifice of invagination, there is most pressing need for a 
re-examination of the asserted coincidence of the mouth with 
that orifice in the case of Actinia and Lumbricus. Finally 
if it should appear that the coincidence does occur in Actinia 
as to mouth, and in Echinoderms and some Molluscs as to 
anus, this would but make the mouth of the anus-less Celen- 
terata identical with the anus of higher animals, in fact, 
it would have to be distinguished as a mouth-anus or or-anus. 
It would, then, seem inconceivable that an animal such as the 
earthworm, having an anus, should also have a mouth iden- 
tical with the orifice of invagination, that is, with the anus of 
closely allied animals. 
The orifice of invagination of those Planule which ex- 
hibit it requires a short and expressive name which shall 
avoid all implications as to its possible relations to mouth 
and anus. I propose to call it the dlastopore (the pore or 
aperture in the blastoderm). ‘This term applies both 
to the orifice which results from the ingrowth of the endo- 
derm in embolé as well as to the gradually narrowing 
margin of small cells which extend over the larger cells in 
epibolé, since embolé and epibolé are but two extreme 
forms of one and the same process and are connected by 
intermediate forms. Another term which I think may 
be used with advantage is that of ‘residual yelk” in place 
of the misleading term ‘‘ food yelk,”’ or nutritive yelk-cells 
or spheres. I have proposed and used this term in m 
paper communicated to the Royal Society in February, 1874, 
to designate that larger and comparatively quiescent portion 
of the egg which becomes differentiated by the separation at 
one of its poles of a more active smaller portion of segregated 
yelk. The ‘residual yelk’’ may be very large and exhibit 
no cleavage, or it may cleave to a limited extent. By the 
use of the term “ residual yelk” we avoid the implication 
contained in the term “ food-yelk,” or “ nutritive yelk- 
spheres,” of an absence in this part of the egg of material 
which can form structure. Not to mention other indications 
of the same fact, the recent observations of Gotte on the 
chick, of Balfour on the shark, of myself on Cephalopods? 
1 T drew attention to the ‘ autoplasts’ of the residual yelk of the Cephalo- 
pods, in February, 1873 (‘Ann. and Mag. Nat. Hist.’). Owsjannikow 
(‘ Centralblatt,’ March 13th, 1875) has recently recognised ‘ autoplasts’ in 
the egg of fishes. 
