GENERAL EMBRYOLOGY 137 



numerous cases, the child's peculiarities hold the mean between those of 

 father and mother; or, in other words, male and female individuals on the 

 average have an equal power of transmitting characteristics. 



Physical Basis of Heredity. — Since in all animals with external 

 fertilization a material connection between parents and offspring can exist 

 only through the sexual cells, these latter must contain the suljstances which 

 render heredity possiijle; further, the two hereditary substances, in cases of 

 equal capacity for transmission, must be present in the egg and in the 

 spermatozoon in equal quantity. By this course of reasoning, the chro- 

 matin which forms the chromosomes has come to be regarded as the bearer 

 of heredity; for we know that the egg contains a great quantity of cytoplasm, 

 but the spermatozoon only the slightest trace of it; that, on the other hand, 

 egg-nucleus and sperm-nucleus furnish equivalent suljstances, and espe- 

 cially the same number of chromosomes, to the cleavage spindles; hence 

 only the chromatin can be regarded as the hereditary substance (idio- 

 plasm). This supports the view expressed before (p. 58) that the nucleus 

 is the bearer of hereditary qualities and determines the character of 

 the cell. 



Theory of Determinants. — These facts of the maturation of the egg and 



spermatozoa and of fertilization have become the starting point for further 

 investigations and associated theories, which in the last few years have acquired 

 great significance. Their relations have also been shown to the extremely im- 

 portant but long forgotten experiments on inheritance in plants by Mendel. 



If we accept the sexual nuclei or their chomosomes as the bearers of heredity, 

 it follows that certain constituents of the chromosomes must contain the anlagen 

 of the characteristics, partly male, partly female, which later develop in the 

 offspring. The simplest setting forth of the connection between the anlagen 

 and the developmental product is Wcismann's 'theory of determinants.' This 

 may be taken as a basis for the following discussion, although objections are 

 brought against it. It represents an organism as a complex of innumerable 

 peculiarities, as a sort of mosaic; and in a corresponding way, the anlagal sub- 

 stance, the chromosome mass (idioplasm) as a similar mosaic of anlagal particles, 

 the determinants. There is a determinant for every paternal or maternal char- 

 acteristic in the offspring, be it prominent or be it latent. The chromosomes 

 must therefore consist of orderly groupings of innumerable determinants. 



Merogony, Artificial Parthenogenesis. — Parthenogenesis shows that since 

 an egg may develop, without the entrance of a spermatozoon, into a complete 

 organism, the chromosomes of the egg are sufficient to produce all of the features 

 necessary for life. This is more clearly shown by artificial parthenogenesis. 

 Many eggs, which normally need fertilization for development, may be stimu- 

 lated to develop by number of chemicals. Similarly the male chromosomes are 

 sufficient for normal development, for if an egs from which the nucleus has 

 been removed be fertilized by a single sperm, it likewise forms an organism with 

 all the necessary features {mcrognny). A fertilized egg must therefore possess 

 duplicate determinants, male and female, for each elementary character, a 

 double assortment of chromosomes. Whether a certain characteristic shall 

 appear purely maternal, or purely paternal in form, or in varving compromises 

 between the two would depend upon the energy of the determinants concerned. 



