INTRODUCTORY REMARKS 3 



by the egg. This is beautifully illustrated in the case of the hybridiza- 

 tion of the egg of the sea urchin with the sperm of the starfish. The 

 development of the pure breed of the sea urchin is after a certain stage 

 (the gastrula stage) typically different from the development of the star- 

 fish, inasmuch as the sea urchin's egg forms at that point a skeleton and 

 goes into the characteristic pluteus form, while the starfish egg forms 

 no skeleton, but undergoes a different development. When the sea 

 urchin's egg is fertilized with starfish sperm, the egg always develops 

 into a pluteus, never into the corresponding starfish form. If we exam- 

 ine the adult forms of hybrids, however, we find that it makes no differ- 

 ence which of the two forms furnishes the spermatozoon or the egg. 

 This difference in the influence of the spermatozoon and egg upon the 

 early embryonic and the adult form of the offspring is possibly due to 

 a difference in the mass of the egg and the spermatozoon, the latter being 

 as a rule much smaller than the former. As soon, however, as the 

 embryo grows and its mass becomes large in comparison with that of 

 the egg, the original difference in the hereditary effects of the two sex 

 cells must diminish or disappear. 



The foundations of a theory of heredity in the adult were laid by 

 Gregor Mendel in his treatise on the hybrids of plants, and this theory 

 is atomistic in its character. He showed that certain simple character- 

 istics of plants, e.g. the round or angular form of the seeds of peas, or 

 the color of their endosperm, must already be represented in the germ 

 by definite determinants. His experiments on the hybridization of 

 various forms of peas indicate that each hybrid contains two kinds of 

 sexual cells, one possessing the determinants for the discriminating 

 fatherly characteristic only, the other for the discriminating motherly 

 characteristic. Both kinds of sexual cells seem to exist in equal numbers 

 in such a hybrid. A similar fact has been discovered in other cases 

 of hybridization, although it does not hold good for all cases. Hugo 

 de Vries and others have begun to build up the physiology of heredity 

 on the basis of Mendel's discovery. 



It is obvious that no theory of evolution can be true which disagrees 

 with the fundamental facts of heredity. It is the merit of De Vries to 

 have shown that a mutation of species can be directly observed in cer- 

 tain groups of plants, and he has further shown that the changes occur 

 by jumps, not gradually. This fact harmonizes with the consequence 

 to be drawn from Mendel's experiments that each individual char- 

 acteristic of a species is represented by an individual determinant in 

 the germ. This determinant may be a definite chemical compound. 

 The transition or mutation from one form into another is therefore 

 only possible through the addition or disappearance of one or more 



