234 Mr. E. R,. Lankester on the [Mar. 12, 



In a paper published in May 1873*, containing the substance of lectures 

 delivered in the preceding October, the author 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 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-lwva,, which swims freely by means of cilia. 

 Lieberkiihn, Miklucho-Maclay, and Oscar Schmidt had previously shown 

 that certain sponges exhibit such an embryonic form; but Professor 

 Hackel described it in many cases, and showed fully its mode of deve- 

 lopment and structure. 



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

 trsea theory "f. The Gastrula form of the Calcareous Sponges is not formed 

 by invagination, but without any opening in the blastosphere making its 

 appearance ; the cells constituting its walls divide into an endodenn 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 Kowalevsky's account of the development 

 of Pelayia and Actinia, in which the formation of a Gastrula by invagina- 

 tion is described, as in the cases already cited among Yermes, Mollusca, 

 and Vertebrata ; on the other hand, we have Allman's observations on 

 the Hydroids, Schultze's on Cordylophora, Kleinenberg's on Hydra, 

 Hackel's on the Siphonophora, and Hermann Foil's on the Geryonidae, 

 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 Foil'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 invagina- 

 tion be regarded as equivalent to those which arise by internal segrega- 

 tion of an endoderm from an ectoderm ? and if so, which is the typical 



* Annals and Mag. Nat. Hist. 1873, si. p. 321. 



t His most recent views on this matter are contained in a pamphlet dated June 7, 

 1873, ' Die Gaetnea-Theorie.' 



