xl 
THE VOYAGE OF H.M.S. CHALLENGER. 
The exact significance of the plasma-like mass which surrounds the spadix, as well 
as the portions which become detached from it, seems open to question. That they 
contain cell-like elements there can be no doubt, and as these show themselves before 
any apparent segmentation can be detected, they can scarcely be otherwise regarded 
than as the remains of the germ-cells, which from an early period filled the gonophore, 
and are now in process of coalescence with one another so as to form the plasma mass, 
while others have probably broken down, and served as nutriment for the developing 
embryo. 
Ciamician, however, regards each plasma mass in Tubular ia mesembryanthemum 
as an ordinary ovum, and compares the cell -like elements which exist in it to the 
“ pseudo-cells ” discovered by Kleinenberg in the ovum of Hydra. These cell-like 
elements are, however, very different in appearance from the hollow thick-walled 
corpuscles described as pseudo-cells by Kleinenberg, while the phenomena presented by 
the development of the embryo in Myriothela (see below, p. xli) are in favour of 
regarding the plasma masses of Tubularia as formed by the coalescence of the original 
germ-cells. In whatever light we view them, the further course of development in 
Tubularia indivisa and Tubularia larynx is as follows : 1 — 
The portion which in the way just mentioned has been detached from the general . 
mass, continues to lie for some time in the cavity of the gonophore, where it becomes 
further developed into an actiniform larva. During this development, it first acquires a 
flattened, somewhat disc-shaped form, and in such shape becomes extended over the 
remaining portion of the sexual mass by which the spadix continues to be surrounded. In 
the centre of the disc a cavity — the primitive digestive cavity — soon shows itself, while 
from the circumference short but thick outbulgings of this cavity radiate, and soon become 
elongated into tentacles. The disc at the same time becomes more gibbous on the side 
turned away from the spadix, while a mouth makes its appearance in the centre of the 
opposite side'. The embryo now retreats from the residual generative mass, the mouth 
is seen to be elevated on a conical prominence (hypostome), while the side opposite to the 
mouth becomes prolonged into the commencement of a stem-like extension which has the 
digestive cavity of the embryo continued into it. In this state it escapes from the 
gonosome, a second circle of very short tentacles having in some species ( Tubularia 
indivisa) become first developed round the mouth, while in others ( Tubularia larynx) the 
oral tentacles do not make their appearance until after the escape of the embryo. The 
embryo continues free for a period, during which it creeps about by the aid of its circlet 
of long proximal tentacles, which are now thrown back round the aboral pole. In the 
meantime the aboral pole has become further elongated into a cylindrical stem which soon 
clothes itself with a perisarc and fixes the young Tubularia to some neighbouring object. 
After the escape of the embryo, or even during its development within the gonophore, 
1 Gymnoblastic Hydroids, p. 90, pi. xxiii. figs. 11-16 and 19-24. 
