896 RECORD OF CURRENT RESEARCHES RELATING TO 



form the mesoderm, probably answering to the internal cell-mass of 

 Halisarca ; the latter become flattened and form the ectoderm. 



At the conclusion of the invagination process, when the blasto- 

 pore has closed up, the embryo consists of an external layer of ecto- 

 derm cells, and an external mass of large cells in which arc seen the 

 developing spicule. The ectoderm cells send out pscudopodia, and 

 confer upon the whole larva the power of executing amoeboid move- 

 ments. While these continue no distinction between the ectoderm 

 cells can be seen, but if the larva is irritated, by touching it with a 

 needle, the pseudopodia are drawn in, and the cell-contours become 

 evident, so that, as in Ascetta, the syncytium theory breaks down. 



In Leiicandra the developmental process is essentially the same ; 

 so that there are two types of development amongst the calcareous 

 sponges, one represented by Ascetta, the other by Sycandra, Leucandra, 

 and Ascandra. 



4. The Ingestion of Nutriment in Sponges. — Metschnikoif directly 

 observed the ingestion of carmine in Halisarca, Ascetta, and Spongilla, 

 by both the endoderm and the mesoderm cells, but in neither case by 

 the ectoderm. In Ascetta, also, the extrusion of particles from a spon- 

 taneously formed aperture in a cell was observed. In Spongilla the 

 cells of the ciliated chambers, like those of the ectoderm, were not 

 observed to take in nutriment ; this has an important bearing on 

 Balfour's theory.* 



In Halisarca it was also observed that when the sponge was over- 

 fed, the endoderm cells increased to such an extent by the ingestion 

 of carmine, that the canals disappeared entirely, and the sponge 

 appeared like a mass of carmine, disk much shorter, containing 

 amoeboid cells, with an ectodermal investment. This phenomenon is 

 interesting as being of the same nature as the obliteration of the 

 alimentary canal during digestion, in certain Turbellarians.f 



5. General Bemarks. — The author mentions having found a Sjyon- 

 gilla in which there was no trace of ciliated chambers, and states 

 further that he has observed, in the young of this species, the cham- 

 bers disappear, and subsequently reappear — the amoeboid or parenchy- 

 matous cells undergoing conversion into ciliated cells. The conclusion 

 deduced from this, that the " endoderm " is formed periodically from 

 " mesoderm," and is therefore a secondary structure, is borne out by 

 the facts of development as seen in Halisarca (Metschnikoif), AplysiUa 

 (Schulze), and other genera, in which the embryo consists of a solid 

 mass of parenchyma with an ectodermal investment. This lends 

 suj)port to the view that the true primary germ-lamellfe are the ecto- 

 derm or epiblast, and a neutral parenchymatous internal layer (lower 

 layer cells of Vertebrata), from which latter the mesoderm or meso- 

 blast, and the endoderm or hypoblast take their rise as secondary 

 formations. 



The author concludes with some criticisms on the gastrula theory 

 of Haeckel and the planula theory of Ray Lankester. He considers 

 that the stage originally succeeding the blastula (a hollow sphere of 



* 'Quart. Joiirn. Micr. Sci.,' January 1S79, and this Journal, antr, p. 177. 

 t This Journal, ante, p. 287. 



