ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 203 



forming genital cells ; these were probably amoeboid, like those of Proto- 

 sjpongia, and, after separation from the wall of the colony, would fall 

 into the genitocoel, as the cavity of the colony may be called. 



We know that, in Volvox, daughter-colonies may give rise to partheno- 

 gonids before leaving the mother-colony, and that they sometimes do so 

 before their orifice closes ; the same may justly be supposed to have 

 happened with the ancestral forms of the Metazoa, and, as it would be 

 an advantage, we should have forms passing the greater part of their 

 existence as open vesicles, strongly charged with parthenogonids ; this 

 open stage is regarded as the prototype of the Metazoa, and is called the 

 Genitogastrula. In the limited space at their disposal, only some of the 

 parthenogonids would arrive at maturity, while others would retain their 

 amoeboid form ; in this way we get genital cells and nutrient cells ; for 

 the inner layer composed of them Prof. Salensky proposes the term 

 Phagogenitoblast, while he retains for the outer layer, formed of amoe- 

 boid cells, the name of Kynublast proposed by Mecznikow. The 

 internal nutrient cells could only function if their colonial orifice 

 remained open, and it would be advantageous, therefore, for it to be so 

 till the moment of the maturation of the sexual products, and the forma- 

 tion of daughter-colonies ; the expulsion of the embryos would necessi- 

 tate the formation of one or more orifices of exit, and it would be a 

 matter of indifference whether or no these corresponded to the primitive 

 opening. 



In the development of the Metazoa the same modifications are seen, 

 and the blastopore is to be regarded as the remnant of the colonial 

 orifice, while the new openings (mouth and anus) are connected phylo- 

 genetically with the appearance of a secondary orifice in the colonies of 

 Flagellates. 



Prof. Salensky thinks that two types have been confounded under 

 the term blastula ; whether it is formed by delamination (schizoblastula) 

 or immigration (poreioblastula), the blastula is palingenetic, and corre- 

 sponds to a closed colony of Flagellates ; but in the Gastroblastula, 

 Amphiblastula, Archiblastula, Periblastula, and Discoblastula, there 

 have been cenogenetic alterations ; the cavity has appeared precociously. 

 The author next discusses the formation of the mesoderm, which is, 

 as he justly says, one of the most obscure problems of comparative 

 embryology ; the more observations have multiplied, the clearer has it 

 become that the mesoderm arises in different ways. To him it appears 

 to be a complication of the primordial diploblastic form of the Metazoa ; 

 this is shown by the existence of Metazoa in which there is no mesoderm, 

 and the development of this layer subsequently to that of the ecto- and 

 endoderm. If we can imagine that, for some reason — the growth of the 

 genitogastrula, for example — the vibratile cells, which served for the 

 locomotion of the organism, were not sufficient for its movement, and 

 that this want brought about a gradual adaptation of the deep amoeboid 

 cells to the needs of locomotion, we should get a contractile layer formed 

 between the ectoderm and the genito-endoderm. It would be indifferent 

 to the organism whether these mesodermic cells arose by immigration of 

 new cells or from cells that had previously immigrated ; in either case 

 there would be a triploblastic gastrula, but in one the mesoderm would 

 appear to have its origin from the endoderm, and in the other case from 

 the ectoderm. 



In conclusion, Prof. Salensky discusses the phylogenetic relations of 



