HYDROID PHASE OF LIMNOCODIUM SOWERBYI. 509 
of undoubted endoderm and ectoderm already mentioned, 
there occurs, round the mouth of the adult polyp internally, a 
little mass of deeply stained non-vacuolated cells of which the 
outlines are only rarely distinguishable. They resemble the 
ectoderm more than the endoderm, but are not exactly con- 
tinuous with either layer, and occur in all specimens of the 
hydroid all through the summer, except in the most recently 
budded examples; they possibly correspond to the “ Glocken- 
kern,’ or primary ingrowth for medusoid gemmation, but 
whether they are ectodermal or endodermal in origin there is 
no evidence to decide. 
The gemmation of a new hydroid polyp presents no very 
special features ; the bud differs from the parent only in the 
uniform cubic shape of the ectoderm cells, in the absence of a 
mouth, of nematocysts, and of the little mass of cells round 
the mouth just mentioned, and in the non-differentiation of 
the endoderm into two regions. The adventitious case of the 
parent is not continued on to the bud, but is acquired by it 
when freed. The bud may either remain attached to the 
parent, or may be nipped off and settle close by, its tissues in 
either case gradually undergoing the differentiations which 
characterise the adult. 
On the gemmation of the medusoid I have unfortunately but 
few details to offer, for reasons already mentioned ; the process 
is diagrammatically represented in fig. 8. In the sole spe- 
cimen available for sections the medusoid was formed at the 
apex of a polyp, and measured about °180 in diameter; in the 
first specimen its relations were not quite clear. The outer 
layer of ectoderm calls for no comment; at the apex it has 
apparently grown inwards to form (1) a solid plug of cells, 
which doubtless represents the preumbral lid of Lankester, 
and (2) separated from this plug by a thin line (? limiting 
membrane), and apparently composed of similar but more 
vacuolated cells, a somewhat globular mass, in which is already 
recognisable an excavation to form the subumbrellar cavity. 
Immediately over the surface of the manubrium these ecto- 
dermal cells show traces of differentiation into the flattened 
