44 PUOCEEDIWGS OF THE 



llaeckelian in his vieu s of the gastrula-or at least of the gerin- 

 layer theory, which he cLii.n.s to he stron-er than ever. If there 

 IS a real hiiuhiiiMMitiilly important separation such as he claims 

 betvveen the germ hiyers, it la quite incouceivabJe that there could 

 be torm(Hl cells of one layer from those of another layer. In 

 regeneration of tissues we have clear evidence that ectoderm can 

 lorm me.stKlerm and endoderm, that endoderm can form ectoderm 

 and mesot erm. Mesoderm is not very happy in its formation of 

 the other layers, but Dendy has shown that in Anledon the endo- 

 derm can come from ectoderm and from mesod<;rm. 



I would altogether dissent from .Sir Eay Lankester's line of 

 evolution from the gastrnla. I am inclined myself at present to 

 regiml the Annelids as coming from some Actinian-like ancestor. 

 In this, as in a 1 Actinia, the secreting digestive epithelium, that 

 ot the stomodtcum and mesenterial filaments, is derived from 

 (grows down from) the ectoderm after the whole of the gastro- 

 vascular cavity is lined by an epithelium which is capable of 

 ingestion but not of extracellular digestion. My own work is 

 not suihciently advanced perhaps for me to make this statement 

 but such were the indications I obtained. The lining epithelium 

 ot the cavity would be equivalent to and homologous with the 

 endoderm ot I/i/dra, and it would form the mesoderm of three- 

 layered animals, the endoderm being an entirely new formation 

 1 am aware that there are great, even insurmountable, difficulties 

 in respect to this view, but the ectoderm and endoderm of higher 

 forms appear to me to be far more intimately related in their 

 functions than are either of them to the mesoderm. 



In the experimental work of Driesch, Wilson, and others, we 

 get into a maze of difficulties in regard to the preformationist 

 hvpothesis. Blastomeres, it is clear, are to a large degree inter- 

 changeable. Incidentally, a fourth blastomere gives a gastrnla in 

 Am2)hio,vus. Again, in budding there are difficulties with this 

 theory, the gut of some budded-olf Polyzoa being formed from 

 mesoderm, while of Tunicates, supposed relations of the Verte- 

 brates, GlaveUma buds from the endoderm and Botryllus from the 

 ectoderm, giving ectoderm and endoderm respectively ; and do not 

 some Sponges turn inside out to give the adult ? 



I need scarcely go further into the question of the germ-layer 

 theory. The confusion when it is applied to Vertebrates is 

 obvious, and we get everywhere involved in difficulties in Inverte- 

 brates. If the gastrnla be a general stage on which great stress 

 IS to be laid, it necessarily might be supposed that the stages up 

 to It should be the same, while actually in the segmenting e^gs we 

 get the most diverse fates for the individual cells. " 



On the whole it is abundantly clear, it appears to me, that it is 

 the nurture as well as the nature of the individual organs which is 

 to be discussed. The law of recapitulation in embrvology has only 

 a limited applicability. Surely the transitory characters are at 

 best only a very partial reminiscence of the structural types 



