DEVELOPMENTAL' HISTOEY OE THE MOLLUSCA. 
33 
Professor Haeckel to the embryonic form which I had proposed to designate by the 
old name Planula ; and the multicellular blastosphere, from which the Gastrula is 
developed, which I had proposed to speak of as a polyplast , he well christens the 
Morula. Professor Haeckel was able to show in his monograph that the Calcareous 
Sponges exhibit a beautifully definite Gastrula- larva, which swims freely by means of 
cilia. Lieberkuhn, Miklucho-Maclay, and Oscar Schmidt had previously shown that 
certain sponges exhibit such an embryonic form ; but Prof. Haeckel described it in 
many cases, and showed fully its mode of development and structure. 
This brings us to an important point in what ITaeckel calls the “ Gastroea theory 
The Gastrula form of the Calcareous Sponges is not formed by invagination. Without 
any opening in the blastosphere making its appearance, the cells constituting its walls 
divide into an endoderm and an ectoderm ; then, and not until then, an orifice is formed 
from the central cavity to the exterior by a breaking through at one pole. Careful 
accounts of the development of Ccelenterata, with a view to determine the mode of 
development of the Planula or Gastrula form in regard to the question of invagination, 
are not to hand in a large number of cases. But, on the one hand, we have Kowxlevsky’s 
account of the development of Pelagia and Actinia, in which the formation of a Gastrula 
by invagination is described, as in the cases already cited among Vermes, Mollusca, and 
Vertebrata; on the other hand, we have Allman’s observations on the Hydroids, 
Schultze’s on Cordylophora, Kleinenberg’s on Hydra, Haeckel’s on the Siphonopliora, 
and Hermann Fol’s on the Geryonidee, in which the ectoderm and endoderm of the 
embryo (which is at first a Planula without mouth, then a Gastrula with a mouth) 
are stated to arise from the splitting or “delamination” of a single original series of 
cells forming the wall of the blastosphere. Hermann Fol’s observations are of especial 
value, since he shows most carefully how, from the earliest period, even when the egg 
is unicellular, its central part has the character of the endodermal cells, its, peripheral 
part that of the ectodermal cells. 
The question now arises, can the Gastrulce which arise by invagination be regarded 
as equivalent to those which arise by internal segregation of an endoderm from an 
ectoderm 1 and if so, which is the typical or ancestral mode of development 1 and what 
relation has the orifice of invagination in the one case to the mouth which, later, breaks 
its way through in the other 1 
It is not within the scope of the present memoir to discuss these questions at length ; 
but I may say that I am of opinion that we must regard the Gastrula- sac, with its 
endoderm and ectoderm, as strictly equivalent (homogeneous, to use another expression) 
in the two sets of cases. One of the two methods is the typical or ancestral method of 
development, and the departure from it in the other cases is due to some disturbing- 
condition. I believe that we shall be able to make out that disturbing element in the 
condition of the egg itself as laid, in the presence in that egg of a greater or less amount 
of the adventitious nutritive material which Edouard van Beneden calls “deutoplasm.” 
* His most recent views on this matter are contained in a pamphlet dated June 1873, ‘ Die Gastrsea-Theorie." 
MDCCCLXXV. F 
