526 



SCIENCE. 



[K. S. Vol. XXI. No. 536. 



tion the normal number is again restored. 

 Van Beneden, Boveri, Hertwig and others 

 have shown in the most convincing manner 

 that these chromosomes are the principal 

 seat of the material substance concerned 

 in hereditary transmission, and recently 

 Boveri has determined that the several 

 chromosomes are individually different in 

 the heritable qualities which they bear. 

 Coincidently with this notable discovery 

 Montgomery found that the reduction of 

 the number of chromosomes in the sex cells 

 is effected by the union of chromosomes 

 into pairs and he showed good reason for 

 believing that one member of each pair 

 came from the father and the other from 

 the mother. This conclusion was con- 

 firmed and extended by Sutton, and he, as 

 well as others, pointed out the suggestive 

 fact that in the maturation divisions im- 

 mediately preceding the formation of the 

 ripe egg or sperm only one chromosome of 

 each pair goes into each mature germ cell. 

 This parental purity of individual chro- 

 mosomes in the sex cells corresponds to the 

 purity of parental characters in the experi- 

 ments of Mendel and the chance combina- 

 tion of these chromosomes into pairs in 

 fertilization corresponds with the combina- 

 tions of qualities (the Mendelian ratio) in 

 alternate inheritance. These remarkable 

 discoveries as to the organization of the 

 nucleus demonstrate that the germ cells are 

 by no means simple and undifferentiated, 

 as has often been affirmed, but rather that 

 they contain numerous visible morpholog- 

 ical elements, each of which has a partic- 

 ular role in hereditary transmission, and 

 they suggest that modifications of these 

 elements, however produced, are the real 

 causes of evolution. 



Much work has also been done upon the 

 differentiations of the cytoplasm in the 

 egg cell (obviously the sperm is unsuitable 

 for such a study) ; with few exceptions the 

 earlier experimental work led to the con- 



clusion that the cell substance was isotropic 

 and undifferentiated. However, many 

 careful observations have shown that in the 

 case of many animals the cytoplasm is 

 visibly differentiated in certain areas of 

 the egg and that the substances of these 

 areas give rise in the course of development 

 to particular organs or parts of the em- 

 bryo. It is now known that in the eggs 

 of a considerable number of animals be- 

 longing to several different phyla all the 

 axes and planes of symmetry of the future 

 individual are marked out in the unseg- 

 mented egg, and in the case of certain anne- 

 lids, mollusks, echinoderms and aseidians 

 it has been discovered that the substances 

 of the ectoderm, the mesoderm and the 

 endoderm are visibly differentiated in the 

 egg before cleavage begins. In certain 

 aseidians I have found that all the prin- 

 cipal organs of the larva, viz., the muscles 

 and mesenchyme, the gastric endoderm and 

 general ectoderm, the nervous system and 

 notochord, are all represented in the two- 

 celi stage by visibly distinct substances 

 which are definitely localized in the egg. 

 These facts show that in certain groups of 

 animals there is such a thing as a morphol- 

 ogy — not merely a promorphology— of the 

 ovum and they demonstrate that there are 

 morphological elements in the cytoplasm 

 lapon which evolutionary forces may act. 



It is a matter of prime imiKtrtance to 

 know whether the nucleus contains the 

 only hereditary material carried over from 

 one generation to another or whether cer- 

 tain characters, such as polarity, symmetry 

 and the localization of organ bases in the 

 egg, may not have their seat in the cyto- 

 plasm. It is as yet too soon to make any 

 positive assertions on this point but the 

 evidence seems to favor the view that the 

 nucleus is at least the principal seat of the 

 inheritance material. Even in eases where 

 the cytoplasm of the egg is so highly dif- 

 ferentiated as in aseidians this conclusion 



