Limitations of the Cell Theory. 79 



forth above, implying that germinal changes arise in a fixed life- 

 cycle through a change in the cell unit which is represented in every 

 nucleus and therefore may modify every stage of the new organism. 1 

 On the other hand, if the more fundamental facts of recapitulation 

 mean anything, they imply that at some time an actual lengthening 

 of the life-cycle has taken place, either hy the addition of cell 

 divisions at its end or by their intercalation at some point. Such 

 a process can not easily be accomplished by a variation in the 

 structure of the cell or nuclear unit itself, but must rather be the 

 result of the organism as it were overcoming its cell shackles and 

 by its own energy producing new developments, though such novel 

 additions are themselves cellular in structure. 



From the time the foundations of the cell theory were laid by 

 Schleiden and Schwann in 1838-39, its universal sway was scarcely 

 questioned for over half a century. During this period it was 

 established that cells arise only from the division of previous cells, 

 and the cell theory culminated in such conceptions as the physio- 

 logical division of labour among cells, the mosaic theory of 

 embryonic development, the individual as the sum of the activities 

 of its various cells. In short, it came to be assumed that cells 

 make the organism, while the contrary fact that the organism after 

 all makes its cells was tacitly or explicitly denied. In the same 

 way it was assumed that each cell of a multicellular organism 

 necessarily corresponded with the whole organism in the Protozoa 



One of the first reactions from this extreme development of the 

 cell theory, which made the organism not a master in its own house 

 but a slave of its constituent cells, was a well known paper by 

 Sedgwick (1894) on the inadequacy of the cell theory of development. 

 His views were based on studies of the embryos of Peripatus 

 capensis, which he believed were essentially cosnocytic in structure; 

 also upon the development of mesenchyme and nervous tissue 

 in Elasmobranch embryos. As far as Peripatus is concerned, 

 delicate cell walls have recently been demonstrated (Glen, 1918) 

 both in the ectoderm and endoderm layers, by careful preparations 

 and the use of an immersion lens. But their demonstration does 

 not affect the fundamental question of the relation of the cell to 

 the organism, for, to mention only a few cases, it is well known 

 that in the formation of the blastoderm of the insect egg, as well 

 as in the early stages of development of the female gametophyte 

 and proembryo in Gymnosperms, a stage is passed through in which 

 1 It appears that, especially in animals, the change is frequently visible 

 or obvious only in certain organs. 



