Further Examination of the Cell-Theory 209 



ill Roux's original theory, are surely features of the earliest 

 stages of ontogeny, even of tlie unsegmented ^gg- A passage 

 from His's Unsere Korperform quoted by Wilson makes this 

 clear. "The material of the genn 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 

 everj' 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 Qgg. According to this principle, the 

 germ-disc contains the organ-germs spread out in a flat 

 plate, and conversely, every point on the germ-disc reap- 

 pears in a later organ. I call this the principle of organ- 

 forming germ-regions.^* ^^ 



But notice now that the organ-forming germ-regions 

 which were Born's "beginning stages" in his grafted larvae, 

 were by no means "germ-regions" of the unfertilized or even 

 fertilized ^gg. They were parts of larvae, i.e., of individual 

 animals well advanced in development. The pieces in his 

 mosaic works were not single cells or parts of cells but great 

 groups of cells, many of them already considerably differ- 

 entiated from one another, but yet so correlated in their 

 activities as to enable the grafted parts of the animal to 

 maintain their specific identity. 



A fundamental question, then, raised by the mosaic theory 

 as formulated to-day is. What is an organ-forming germ 

 area.? or, more briefly. What is germinal material.'^ Is it 

 the material of each of the first two, or in some instances 

 four blastomeres as indicated by the frog's ^gg? Is it 

 nuclear material as conceived by Roux's modification of his 

 original theory? Or is it in accordance with Born's idea, 

 the material of any group of cells no matter how large the 

 group and how many kinds of cells in it, so long as the 

 group is able to develop true to the kind of organism to 



