40 



EMBRYOLOGY 



QUALITATIVE DIFFERENCES 

 QUANTITATIVE DIFFERENCES 



Fig. 18. Contrasting theories of the organization of the protoplasm of the 

 egg. Left: E represents a substance or a factor. The concentrations of E are 

 represented in graded fashion by the symbols E N > £ A -i > E N . 2 . . . E Q . 

 Right: S ec , S en , and £„, are three different chemical substances responsible for 

 the formation of ectoderm, endoderm, and mesoderm of the embryo. 



These two theories are summarized in the form of a diagram in Figure 

 18 and may be contrasted something as follows. The quantitative theory 

 assumes that the same substance or factor, called E, is present throughout 

 the egg, but at the animal pole, for example, it will be in concentration E„. 

 In some other region the concentration will drop to E n —\. And at the vegetal 

 pole E will be present in the lowest concentration, Ea. The differences in 

 development are thus caused by differences in quantity of E. 



The qualitative theory assumes that there are actually different sub- 

 stances in the parts of the egg. One substance we might call S ectoderm , another, 

 Sendoderm, and a third, S me80derm . These three substances transform the parts of 

 the egg into the ectoderm, endoderm, and mesoderm of the embryo. This 

 theory is a refinement of the preformation theory. The parts of the embryo 

 are assumed to be present in the egg in the form of morphogenetic sub- 

 stances. 



The most reasonable compromise between these two theories is that 

 the parts of the egg may at first be quantitatively different, but that some 

 time later they become qualitatively different. That is to say, in the course 

 of development different substances must arise which insure the differentia- 

 tion of various parts of the egg. One such compromise theory already has 

 been formulated for the development of the sea urchin egg. It supposes that 



