On the Developmental History of the Mollusca. 83 



groups of the animal kingdom. At the end of the year 1872, 

 Professor Hackel's splendid Monograph of the Calcareous Sponges 

 appeared, in which the same questions are methodically discussed. 

 The name Gastrula is given by Professor Hackel to the embryonic 

 form which the author proposed to designate by the old name 

 Planula ; and the multicellular blastosphere, from which the 

 Gastrula is developed, which the author had proposed to speak of 

 as a Polyplast, he well christens the Morula. Professor Hackel 

 was able to show in his monograph that the Calcareous Sponges 

 exhibit a beautifully definite Gastrula-\a,rva, which svvims freely by 

 means of cilia. Lieberkuhn, Miklucho-Maclay, and Oscar Schmidt 

 had previously shown that certain sponges exhibit such an em- 

 bryonic form ; but Professor Hackel described it in many cases, 

 and showed fully its mode of development and structure. 



This brings us to an important point in what Hackel calls the 

 " Gastraea theory "*. The Gastrula form of the Calcareous Sponges 

 is not formed by invagination, but, without any opening in the blasto- 

 sphere 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 deve- 

 lopment of Ccelenterata, with a view to determine the mode of deve- 

 lopment 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 Kowalevsky's account of the develop- 

 ment 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 Cordylo- 

 phora, Kleinenberg's on Hydra, Hackel's on the Siphonophora, 

 and Hermann Poll's on the Geryonidfe, 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 Poll's observa- 

 tions 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 Gastrula which arise by in- 

 vagination be regarded as equivalent to those which arise bv in- 

 ternal segregation of an endoderm from an ectoderm ? and if so, 

 which is the typical or ancestral mode of development, and what 

 relation has the orifice of invagination in the one case to the 

 mouth which, later, breaks its way through in the other ? 



It is not within the scope of the present memoir to discuss these 

 questions at length ; but the author is of opinion that we must 

 regard the Gastrula-s&c with its endoderm and ectoderm as strictly 



* His most recent views on this matter are contained in a pamphlet dated 

 June 7, 1873, ' Die Gastrica-Theorie.' 



0* 



