DEVELOPMENT 207 



74. ON THE NATURE OF THE DEVELOPMENTAL 



PROCESS 



Any discussion of this matter is necessarily very largely historical 

 — there is, at present, no working hypothesis of development and, 

 at the most, we can only speculate, with very obvious limitations. 

 The oldest, and still the prevailing conception is one that contains 

 the idea of involution. An ovum, seed, bud, etc., does not appear 

 to be the organism which it will become in the course of its 

 development : nevertheless there is in it, somehow or other, that 

 which will evolve, unfold, or develop into a particular kind of 

 organism. This was the notion of development commonly held 

 up to the time when the morphological method (because of the 

 invention of the microscope) was applied to the study of embry- 

 ology. When, however, it could easily be seen that there was 

 nothing in, say, the blastoderm of the developing chick, in the 

 least resembling the organs of the fowl the notion of involution 

 was, for a time, abandoned in favour of that of epigenesis. From 

 the relatively structureless blastoderm came the complex system 

 of parts of the embryo : these grew up upon the blastoderm, 

 apparently because of the action of the environmental agencies. 

 But obviously the conception could not be maintained because it 

 was easy to see that different blastoderms, exposed to the same 

 external agencies gave rise to different kinds of organisms. It 

 was necessary to postulate '' internal factors " as well as external 

 ones in a developmental process, and by and by these internal 

 factors were apparently found when the complex architecture of 

 the *' germ-plasm " was revealed by the perfection of the com- 

 pound microscope. So notions of involution, based on the 

 morphology of the nucleus, w^ere again applied to the investigation 

 of development and such is the prevalent outlook at the present 

 time. 



The notion of '^ representative particles. ^^ This is very old, but 

 it assumed modern form in the hypothesis oi pangenesis elaborated 

 by Charles Darwin. From all parts of the body of the organism 

 particles were given off which had, in some way, the potentialities 

 of the organs, or parts, from which they were derived. These 

 representative particles became lodged in the cells of the gonads 

 — the ova and spermatozoa — and when the embryogeny began 

 the potentialities of the particles became realized in the organ- 

 anlagen and tissues. The notion amplified the older one of 



