THE THEORY OF GERMINAL LOCALIZATION 



been advocated by some of the foremost students of development. 

 It is maintained that, although the embryo is not pre-formed in 

 the germ, it must nevertheless be predetermined in the sense that the 

 egg contains definite areas or definite substances predestined for the 

 formation of corresponding parts of the embryonic body. The first 

 definite statement of this conception is found in the interesting and 

 suggestive work of Wilhelm His ('74) entitled Unsere Kb'rperform. 

 Considering the development of the chick, he says : " It is clear, on 

 the one hand, that every point in the embryonic region of the blasto- 

 derm must represent a later organ or part of an organ, and on the 

 other hand, that every organ developed from the blastoderm has 

 its preformed germ (" vorgebildete Anlage ") in a definitely located 

 region of the flat germ-disc. . . . The material of the germ is 

 already present in the flat germ-disc, but is not yet morphologically 

 marked off and hence not directly recognizable. But by following 

 the development backwards we may determine the location of every 

 such germ, even at a period when the morphological differentiation 

 is incomplete or before it occurs ; logically, indeed, we must extend 

 this process back to the fertilized or even the unfertilized egg. 

 According to this principle, the germ-disc contains the organ-germs 

 spread out in a flat plate, and, conversely, every point of the germ- 

 disc reappears in a later organ ; I call this the principle of organ- 

 forming germ-regions." 1 His thus conceived the embryo, not as 

 prQ-forvted, but as having all of its parts prt-localized in the egg- 

 protoplasm (cytoplasm). 



A great impulse to this conception was given during the following 

 decade by discoveries relating, on the one hand, to protoplasmic 

 structure, on the other hand, to the promorphological relations of the 

 ovum. Ray Lankester writes, in 1877: "Though the substance of a 

 cell 2 may appear homogeneous under the most powerful microscope, 

 it is quite possible, indeed certain, that it may contain, already formed 

 and individualized, various kinds of physiological molecules. The 

 visible process of segregation is only the sequel of a differentiation 

 already established, and not visible." 3 The egg-cytoplasm has a defi- 

 nite molecular organization directly handed down from the parent ; 

 cleavage sunders the various " physiological molecules " and iso- 

 lates them in particular cells. Whitman expresses a similar thought 

 in the following year : " While we cannot say that the embryo is 

 predelineated, we can say that it is predetermined. The * Histo- 

 genetic sundering ' of embryonic elements begins with the cleavage, 



1 I.e., p. 19. 



2 It is clear from the context that by " substance" Lankester had in mind the cytoplasm, 

 though this is not specifically stated. 



