34 GENESIS OF MAN. 



lateral pressure against one another. The new form is denomi- 

 nated the Blastosphaera or vesicula blastodermica, while the cellular 

 layer bears the name of germinal membrane (Keimhaut) or blasto- 

 derm. 



The blastosphaere is a stage of embryonic development which is 

 common to all creatures that have a higher organization than that 

 of the synamoebian societies of cells. In many of the lower forms 

 of life it becomes a stage of metamorphosis rather than of embry- 

 onic development, since these minute blastosphaeres lead independ- 

 ent lives for a time, as the larvae of higher forms. This is the case 

 with the calcareous sponges, with many zoophytes, worms, ascidi- 

 ans and molluscs. Such larvae are called Planulce. They are 

 usually covered with cilia, which serve as aids to locomotion. 

 These facts alone would justify the believer in the dependence of 

 ontogeny upon phylogeny in maintaining that this stage had once 

 formed the highest plane of development, and that there had once 

 existed a race of creatures which, after passing through the three 

 preceding stages, completed their career as true blastosphaeres, and 

 that all higher organisms must, in that sense, be descendants of 

 such a race. Haeckel assumes such a group of creatures, which 

 he calls the Planaeada. This hypothesis, however, was scarcely 

 necessary, from the fact that there are animals well known to 

 science which conform in their general structure entirely to this 

 stage of development. Many such creatures now exist, both in 

 the sea and in fresh water, consisting of a single exterior layer of 

 cells, surrounding a fluid or gelatinous interior, and usually pro- 

 vided, like the larval forms, with locomotive cilia. Especially may 

 be mentioned the Synura in the Volvocinae and the Magosphaera 

 planula. The latter was discovered and named by Haeckel, who 

 has carefully traced its development through the lower stages, and 

 proved the Planula to be its highest and mature condition. Such 

 an animal is, therefore, a true Planaea, as strictly so as are the 

 members of Haeckel's theoretical ancestral group of Planaeada. 



To the philosophical embryologist, the blastosphaere-stage pre- 

 sents an extraordinary interest. Nothing could illustrate this 

 better than the remarkable utterance which it has elicited from Von 

 Baer himself, one of the few of his statements which possess not 

 only an ontogenetic, but also a phylogenetic significance. " The 

 farther we go back," says he, "the greater agreement do we find, 



