CHAPTER V 



GhemodifFerentiation 



In the foregoing chapters we have reached the conclusion that, 

 at the beginning of development, the egg protoplasm is of a 

 practically homogeneous constitution, and has a very low degree 

 of multiplicity, though there is a polar and bilateral structure, 

 governed by the axial and cortical gradient systems. We shall 

 now see how the spatial multiplicity of the structure increases 

 in the course of development. 



Several investigations have made it plain that in many cases 

 the first step in this process consists of a local accumulation 

 of substances that previously were evenly distributed over the 

 egg. These accumulations exert a definite influence on the 

 fate of those parts of the germ in which they occur. Originally 

 it was assumed that they supplied the material for certain 

 organs of the future embryo, and for this reason they have been 

 named ''organ- forming substances''. Later this view proved to 

 be far too schematic. The substances accumulated in certain 

 parts of the germ do exert a determining influence on the 

 further development of these parts; they cannot, however, be 

 regarded simply as building material for the organs concerned. 

 It is better, therefore, to call them ''determining substances" . 



Penners' work (1922-25) on the development of the worm 

 Tubifex may be cited as a first example. Soon after fertilisation, 

 accumulations of a special protoplasmic material occur both 

 at the vegetative and at the animal pole of the eggs of this 

 species. These masses are called the vegetative and animal 

 pole-plasm (Fig. 17 a). Experiments by Lehmann (1940) have 

 shown that this accumulation is probably caused by attractions 

 exerted by certain parts of the egg cortex on the material 

 concerned. For even in centrifuged eggs, in which the distribu- 



