32 
MR. E. RAT LANKESTER ON THE 
General considerations relative to the observations contained in the preceding 
Contributions (Nos. I., II., III.). 
Before leaving the preceding records of observations to the consideration of the reader, 
I may point out briefly their bearing on two matters of theoretical importance, viz. (1) 
the origin and significance of what has been called the Gastrula phase of development, 
and (2) the homologies or homogenies (as I should prefer to say) of the shells, ligaments, 
and internal pens of the Mollusca. More facts have to be sought out and brought to 
bear on these questions ; but whilst occupied in that further search, let me indicate the 
anticipations which must guide and stimulate it. Before doing so I must mention that 
there are a variety of other matters of interest in the facts recorded in the preceding 
pages which cannot yet be brought into any theoretical structure, but which I have not 
on that account kept back, as they will probably be of some service in their isolated 
condition. 
(1) Kowalevsky was the first to describe, in a precise manner, the formation of the 
foundations of the alimentary tract in a developing embryo by invagination of the wall 
of a simple primitive blastosphere, or hollow ball of embryonic cleavage-corpuscles. 
He detected this mode of development in Amphioxus , and subsequently in Ascidia. By 
later researches he was able to indicate the same mode of development in certain Vermes 
( Sagitta , Lumbricus ); and he mentioned incidentally that he had observed a similar 
development in the Heteropodous mollusk Atalanta. I was at this time studying 
the development of Pisidium and Limax , and obtained evidence of the invagination of 
the primitive blastosphere in those two widely separated mollusks. Subsequently at 
Naples I found the same process occurring in Nudibranchs. The probable identity of 
this process of invagination with that so well known in the Batrachians, especially 
through Stricker’s admirable work on the subject, became clear, to those occupied with 
embryological studies, from the facts established by Kowalevsky ; and the “ anus of 
Ruscoui ” co aid now be recognized in the “ orifice of invagination ” present in members 
of the three large groups of Vermes, Mollusca, and Vertebrata. 
The embryonic form produced by this invagination-process is a simple sac, composed 
of an ectoderm and endoderm, with an orifice connecting the exterior with the cavity 
lined by the endoderm. It, in short, presents the typical structure of the simplest 
Coelenterata, and corresponds exactly with the so-called Planula of the polyps and 
corals. Hence we are tempted to see in this primitive invagination-form the repre- 
sentative of the Ccelenterate phase of development of the whole animal kingdom. In 
a paper published in May 1873*, containing the substance of lectures delivered in the 
preceding October, I have discussed this notion at some length, and other points 
connected with the attempt to work out the correspondences of the embryonal cell- 
layers of the various groups of the animal kingdom. At the end of the year 1872, 
Professor Haeckel’s splendid Monograph of the Calcareous Sponges appeared, in which 
the same questions are methodically discussed. The name Gastrula is given by 
* Annals & Mag. Nat. History. 
