398 INHERITANCE AND DEVELOPMENT 



definite areas or definite substances predestined for the formation of 

 corresponding parts of the embryonic body. The first clear state- 

 ment of this conception is found in the interesting and suggestive 

 work of Wilhehn His ('74) entitled Unsere Kbrperform. Considering 

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

 that every point in the embryonic region of the blastoderm must rep- 

 resent 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 back- 

 wards 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 fiat plate, and, 

 conversely, every point of the germ-disc reappears in a later organ ; 

 I call this the prijiciple of organ- forming germ-regions^^ His thus 

 conceived the embryo, not as ^r&formed, but as having all of its parts 

 i^relocalized \n the egg-protoplasm (cytoplasm). 



A great impulse to this conception was given during the follow- 

 ing 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 individiialized, various kinds of physiological molecules. The 

 visible process of segregation is only the sequel of a differentiation 

 already estabhshed, and not visible. "^ 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 pre- 

 delineated, we can say that it is predetermined. The ' histogenetic 

 sundering ' of embryonic elements begins with the cleavage, and every 

 step in the process bears a definite and invariable relation to antece- 

 dent and subsequent steps. ... It is, therefore, not surprising to 

 find certain important histological differentiations and fundamental 

 structural relations anticipated in the early phases of cleavage, and 

 foreshadowed even before cleavage begins." * It was, however, Flem- 



' /. c, p. 19. 



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

 though this is not specifically stated. ^ '^^^ p, j^^ 4 '78, p. 49. 



