2 1 2 REGENERA TION 



layers, if not in the sense of a repetition of an ancestral adult gastraea, 

 yet in the sense that similarity in embryonic development may in 

 some cases find its historical explanation in a common descent. 



If in the light of this discussion we turn to the phenomena of 

 regeneration, we again find evidence showing that the germ-layer 

 theory fails to apply in all cases. It has been pointed out that in 

 lumbriculus, and in the naids, the new mesoderm is derived from the 

 ectoderm, and does not come from the old mesodermal tissues. The 

 mesoderm of the embryo in annelids is derived from one, and later 

 from two, superficial cells of the blastula, 1 that push in about the time 

 of gastrulation. They cannot, at this time, be referred to one layer 

 rather than to the other. It cannot be affirmed, therefore, that in 

 regeneration, the mesoderm arises from a different layer from that in 

 the embryo, but neither can this be denied. The most important 

 point in this connection is that the new mesoderm comes from the 

 ectoderm that is already differentiated, and not from the mesodermal 

 tissues. It is clear, however, that while the lining of the pharynx in 

 the embryo is ectoJermal, it is endodermal in the regenerated part. 



It is true that these cases are very exceptional, and that generally 

 the new organs come from similar organs in the old part, but one 

 established exception is sufficient to show that the traditional concep- 

 tion of the germ-layers may be of little value, and since the hypothe- 

 sis itself, out of which the idea in regard to regeneration from definite 

 germ-layers has been formed, has been proven to be insufficient in 

 other directions, the time is ripe to look for a more secure footing. 

 It need hardly be added that the idea of a supposed necessity for an 

 organ to arise from a definite germ-layer is so empty of all signifi- 

 cance that we may well rejoice to be able to set it aside as a naive 

 view that has had its day. Furthermore, a new series of problems 

 has arisen in connection with the experimental work to be described 

 in a later chapter. If, as seems probable, the question of the germ- 

 layers will be merged into the much broader question of the origin 

 of the specification of the tissues, we can in the future more profitably 

 direct our attention to the experimental evidence that bears on the 

 latter question. 



THE SUPPOSED REPETITION OF PHYLOGENETIC AND ONTOGENETIC 

 PROCESSES IN REGENERATION 



It has been claimed that at times ontogenetic, and even phylo- 

 genetic, processes are repeated during regeneration. Fraisse, for 

 instance, who advocates this point of view, thinks that it has been 



1 A small amount of embryonic mesenchyme may come from some of the ectodermal 

 quartettes of the embryo and produce the branching muscles of the head, but not the char- 

 acteristic muscles of the trunk. 



