432 SCIENCE PROGRESS. 



Vosmaer (1887, p. 481) while considering the sponges as 

 Metazoa. but not coelenterates, prefers to leave the question 

 of the germ layers undecided. 



Balfour thought that "the germinal layers of the sponges 

 very probably do not correspond physiologically to the 

 germinal layers of other types" (1885, vol. ii., p. 345) 

 whether or not he regarded them as corresponding geneti- 

 cally, i.e. as homologous, is not very clear. The great 

 contradictions which were presented by the facts of sponge 

 development as then known, probably prevented him from 

 expressing a definite opinion, for he hints that there are 

 " strong arguments for regarding the Porifera as a phylum 

 of the Metazoa derived independently from the Protozoa". 

 In his discussion of the development of Sycon however 

 (1879) he certainly seems to regard the two layers of the 

 amphiblastula larva as corresponding to those of other 

 Metazoa, but as becoming reversed in position at the meta- 

 morphosis, the "locomotor and respiratory" ciliated cells 

 passing to the interior, the amoeboid nutritive cells to the 

 exterior. 



Maas (1893) puts forward the view that sponges are 

 derived from two layered Metazoan ancestors composed 

 of ectoderm and endoderm, which have taken on a course 

 of development diverging from all other Metazoa, in that 

 the germ layers have become reversed in position, the true 

 ectoderm being internal in the adult, and the true endoderm 

 external. 



Heider (1886, p. 230) considers sponges "true Metazoa, 

 and the two primary layers of this group as really homol- 

 ogous with those of higher types," but they are to be 

 separated from the coelenterata, and regarded as an in- 

 dependent type. Only the blastula and gastrula stages in 

 development are common to both (p. 233). 



Goette (1886) also believes that "in the embryonic de- 

 velopment of sponges we meet with the blastulze and 

 gastrulae common to all Heteroplastids, as the modified 

 ancestral forms of this group also" (p. 51). He thinks, 

 however, that the apparent contradictions in sponge em- 

 bryology can only be reconciled on the theory that the 



