XXXV111 
THE VOYAGE OF H.M.S. CHALLENGER. 
adult. By further changes in the relative lengths of the diameters, an increase in the 
number of tentacles, and the formation of marginal sense organs and of a velum, the 
larva assumes the form of the adult. 
Metchnikoff 1 and Fol 2 have both, independently of one another, traced the develop- 
ment of the egg in Geryonia hastata, another member of the group which never passes 
through a polypoid trophosome, and have arrived at essentially similar results. In this 
Medusa the segmentation of the egg gives rise to a blastosphere with a large segmentation 
cavity, and with walls as yet formed by a single layer of cells. By the transverse division 
of these cells, the walls of the hollow blastosphere becomes differentiated into an ecto- 
dermal and an endodermal layer. Between these a clear homogeneous gelatinous excretion 
is deposited as the rudiment of a gelatinous umbrella, and by its unequal accumulation 
renders the segmentation cavity more and more excentric, until its endodermal lining 
again comes in contact at one point with the ectoderm. At this point a mouth is formed, 
and the segmentation cavity, now become the gastral cavity of the Medusa, is brought 
into connection with the exterior. Round the mouth the ectoderm forms a thickened 
ridge from which the rudiments of the tentacles shoot forth, and from which a perforated 
diaphragm extends towards the axis as a velum. In none of these cases, however, do the 
observers give any account of the formation of the gastrovascular canals, and a very 
important point in the development has thus been left unsolved. 
The endoderm and ectoderm of the Hydroida represent, as was first pointed out by 
Huxley , 3 the inner and outer germinal layers, to the formation of which the early stages 
of the development of the ovum lead throughout the whole of the Metazoal members of 
the animal kingdom ; but while these become in the higher Metazoa so transformed as to 
be no longer recognisable as definite layers, they remain as permanent elements in the 
structure not only of the Hydroida but of all the other members of the Coelenterata, 
where they still continue as the endoderm and ectoderm of the adult. 
The development of the Hydroid colony through the formation of a planula is char- 
acteristic of the great majority of the Hydroida. There are, however, three genera — 
Tubulctria, Myriothela, and Hydra — in which the development of the egg has been 
proved not to result in the formation of a free planula, while one or two others may 
possibly come into the same category, though an opportunity of studying them sufficiently 
to render this point certain has not yet been afforded. 
Development of the Egg in Tubularia. — In Tubularia and in Myriothela there is 
formed instead of a planula a locomotive actiniform larva which is destitute of a ciliated 
epithelium and moves about by the aid of long tentacular processes. The development of 
the embryo in two species of Tubularia ( Tubularia indivisa and Tubularia larynx) has 
been followed by myself, while Ciamician and Weismann have examined it in Tubularia, 
1 Metchnikoff, loc. cit. 2 Fol, Jenaisclie Zeitschr., Ba. vii. 
3 Huxley, On the Anatomy and Affinities of the Medusae, Phil. Trans., 1849. 
