194 PROFESSOR E. RAY LANKESTER. 
On Youne Sraces of Limnocopium and Geryonta. By 
E. Ray Lanxester, M.A., F.R.S., Jodrell Professor of 
Zoology in University College, London. With Plate 
XIII. 
METSCHNIKOFYS, in a paper in the ‘ Zeitsch. wiss. Zoologie,’ 
1874, p. 17, describes and figures the development of 
Geryonia from the egg up to the formation of tentacles and 
an umbrella cavity. It is, however, obvious from his figures 
and description that certain steps in the development were 
not followed by him with precision—and some of his ob- 
servations are supplemented, whilst others are contradicted, 
by those of Fol (‘ Jen. Zeitsch.,’ vol. vii). It appears to be 
certain, from the observations of both Metschnikoff and Fol, 
that in Geryonia the endoderm forms by delamination — 
the embryo being, when it has so formed, a nearly spherical 
“ diblastula”’ without orifice of any description (Metsch- 
nikoff’s plate ii, fig. 7). The outer layer now grows away 
from the inner, which remains in the form of a lenticular 
sac, closely applied to one pole of the ectodermal sphere. 
From this point forwards the accounts of Haeckel (‘ Jenaische 
Zeitschrift,’ vol. ii), of Metschnikoff, and of Fol, are diverse. 
A mouth is formed by a breaking through into the endoder- 
mal sac, and a sub-umbrellar cavity is formed, and also a 
velum and tentacles ; but the accounts given of the mode 
of origin of these parts is not conclusive in any one of the 
authors’ memoirs above cited. Haeckel observed Geryonia 
embryos, in which he definitely states that the sub-umbrellar 
cavity existed in the form of a closed sac. Metschnikoff 
thinks this an erroneous observation, and due to a mistaking 
of the endoderm sac for the sub-umbrellar cavity. According 
to Metschnikoff, the ectoderm and endoderm are ruptured at 
the central point where the enclosed endoderm sac is resting 
on the inner wall of the larger ectodermal sac, and the 
ectoderm is invagimated to form the sub-umbrellar cavity. 
At the same time a ring grows up at some distance from the 
mouth thus formed, which becomes the margin of the 
umbrella, and gives off, on its adoral side, the velum; on its 
aboral side, the first tentacles. 
Fol’s account of the formation of the sub-umbrellar cavity 
substantially differs from that of Metschnikoff in that he 
does not derive the sub-umbrellar cavity from an invagination, 
1 It is not possible to reject this well-established fact, as the brothers 
Hertwig do, apparently without hesitation (‘ Coelomtheorie,’ 1881). 
