HAECKEL^S RECENT ADDITIONS TO G ASTR^A-THEORY. 53 



and 'Proceedings of the Royal Soc./ 1874), and further, 

 in relation to the development of organs in the middle layer 

 of the germ which are palingenetically hypoblastic or 

 epiblastic, as, for instance, the notochord of Vertebrates 

 (shown by Mr. Balfour to develop from the hypoblast in 

 the sharks, probably its true palingenetic position), and the 

 nerve-ganglia of Loligo (shown not to develop from epiblast 

 as nerve-centres should do in undisturbed palingenesis ; see 

 ' Quart. Journ. Micr. Sci.,' 1875, p. 46). 



As Haeckel observes, first and foremost in this connection 

 are the * wandering ' of cells and shifting of groups of cells in 

 the earliest stages of development — small shiftings taking 

 place in an early stage — such, for example, as the interpene- 

 tration in limited points of the two primitive cell layers, the 

 ectoderm and endoderm, or epiblast and hypoblast which 

 constitute the critical developmental form of all higher 

 animals, the Gastrula or Planula. To this heterotopic move- 

 ment of cells in an early phase of development, possibly 

 occurring even in the first stages of cell-formation or cleavage of 

 the egg, it is very possible that large dislocations in the seat 

 of development of organs, manifesting themselves at a later 

 period, should be ascribed. Haeckel would thus account for 

 the assumed mesoblastic origin of the sexual glands in certain 

 of the higher animals, holding, as do most embryologists, that 

 these glands are palingenetically not part of the middle layer of 

 the germ, but part cither of the epiblast or of the hypoblast. 



Considerations of the same kind enable us to explain in a 

 general way the diversity of origin which the middle layer 

 presents in different groups of animals. At the same time it 

 must be admitted that accurate information as to even the 

 apparent origin of the mesoblast in a large number of cases is 

 not to hand, and the statements of authors worthy of credence 

 vary in reference to one and the same animal. Haeckel, as 

 is well known, holds that the mesoblast is phylogenetically 

 formed of two layers, respectively the deep layer of the 

 ectoderm and the deep (i. e. sub-epithelial) layer of the endo- 

 derm. Its apparent development entirely from hypoblast or 

 endodermintheYertebrates he regards as duetotheheterotopy 

 of the ectodermal portion,^ and the splitting which occurs 



^ A possible explanation of such a case as the disappearance of the ecto- 

 dermal factor of the mesoblast is suggested (I venture to submit) by a 

 comparison with the phenomena of atrophy, such as we observe in the 

 cases of rudimentary organs and the final suppression cf such organs. 

 Atrophy very generally is accompanied by a transference of the nutri- 

 tion proper to the atrophied part to a neighbouring or substituted 

 structure, and when the transference reaches its full development and extends 

 back, into the period of early embryonic life, even the primitive cells which 

 formed the first outlines of an organ or of a germ-layer may be totally 



