GENEOLOGICAL. 375 



organs can be traced back to a few cells specifically 

 predestined from their first appearance, rather than 

 to a homogeneous germinal layer." * In fact, the 

 germ-layer-theory is now regarded by many experts 

 as " inadequate and misleading," and it is being re- 

 placed by a more detailed study of cell-lineage in 

 which segmentation-cells or blastomeres are traced 

 from their origin to their final result. 



Gastrcea-Theory. The same kind of remark must 

 be made in regard to Haeckel's famous Gastroea- 

 Theory (1874). In this there are two propositions, 

 (1) that the gastrula-embryo (the two-layered sac) 

 is of general occurrence, though often disguised, in 

 the development of animals; and (2) that the hypo- 

 thetical ancestral form of multicellular animal (the 

 Gastrsca) was a two-layered sac like a gastrula. 

 But it requires extraordinary ingenuity to find the 

 gastrula-stage in, let us say, the development of a 

 hedgehog, or even in that of the chick. And 

 even when the gastrula is plain, as in starfishes, it 

 is not always clear that its layers are homologous 

 with those of other gastrulae, e.g., in Sponges. As 

 to the other part of the Gastrsea-Theory, there are 

 three or four plausible hypotheses in the field as to 

 the possible form of the ancestral multicellular 

 animal. It is likely enough that there were several 

 forms. 



Recapitulation-Doctrine. Once more, to take the 

 largest generalisation of nineteenth-century embryol- 

 ogy, the Recapitulation-Doctrine or biogenetic 

 law, which suggests that the individual develop- 

 ment is in some measure a recapitulation of the 

 racial history, there are few modern embryologists 

 who regard it without hesitation and suspicion. 

 * Science of Life, p. 131. 



