Physiological Morphology 101 



of preformed germ-regions gives no answer to the question, why 

 some parts of the embryo grow faster than others. Neverthe- 

 less, it is not necessarily in opposition to the theory of growth 

 offered in the preceding chapter. Starting with the idea of His, 

 we may well imagine that the different regions of the ovum 

 are somewhat different chemically, and that these chemical 

 differences of the different germ-regions determine the differ- 

 ences of growth in the blastoderm. Thus the phenomena of 

 heteromorphosis would show that, in some animals at least, the 

 arrangement of preformed germ-regions may be changed by 

 gravitation, light, adhesion, etc. 



It must be asked, however, what, from the standpoint of 

 causal morphology, determines the arrangement of the different 

 germ-regions in the egg. If we answer ''heredity," causal 

 morphology can make no use of such an explanation. Our 

 blood has the temperature of about 37°, but although our 

 parents had the same temperature, the heat of our blood is not 

 inherited, but is the result of certain chemical processes in our 

 tissues. Still it may be possible that the molecular forces of 

 the chemically different substances of the egg determine a 

 separation of these substances and thereby give rise to the 

 chief directions of the future embryo. 



Driesch has shown^ that by shaking a sea-urchin's egg in the 

 four-cell stage the four cells may be separated, and each one be 

 capable of giving rise to a complete embryo, which differs only 

 in size from the normal embryo. If the theory of preformed 

 germ-regions with its later modifications were true, we should 

 expect that every one of the isolated cells would give rise to 

 one-fourth of an embryo. But it has been said that the arti- 

 ficial isolation of one cleavage-cell causes a process of post- 

 generation or regeneration. Driesch, moreover, changed the 

 mode of the first cleavage by submitting the ovum to one-sided 

 pressure. In this way the nuclei were brought into somewhat 



1 Zeitschrift f. wissensch. Zoologie, LIII, LV. 



