96 



SCIENCE 



[N. S. Vol. XXVII. No. 681 



Boveri also discovered that enucleated egg 

 fragments of Echinus fertilized by sperm 

 of Strongylocentrotus produced larvas 

 which had purely paternal characteristics. 

 These results have, however, been called in 

 question by Seeliger, Morgan and Driesch; 

 but even accepting Roveri's results, this 

 experiment does not completely demon- 

 strate that the chromosomes are the only 

 "bearers of the inheritance material," as 

 we shall see later. Finally, as a fitting 

 climax to this chapter of remarkable dis- 

 closures as to the chromosomes, may be 

 mentioned the epoch-making discovery of 

 McClung, Wilson and Stevens that in cer- 

 tain groups of insects the dimorphism of 

 the sexes is correlated with a chromosomal 

 dimorphism of the spermatozoa. 



These are but a few of the many notable 

 discoveries which have been made within 

 recent years regarding the chromosomes, 

 and while they do not demonstrate the 

 truth of the chromosomal inheritance the- 

 ory, they do prove the very great signifi- 

 cance of the chromosomes in the process of 

 heredity. Boveri concludes his last great 

 work on "The Development of Dispermic 

 Sea-urchin Eggs" with these words (p. 

 260): 



After all has been said, I believe that we may 

 regard the view that specific characteristics are 

 transmitted from the parent to the child through 

 the chromosomes of the egg and the sperm nuclei, 

 as a theory which has a series of facts in its 

 favor and up to the present time not a single one 

 against it. 



VI 



Against the extreme form of this theory, 

 as held for example by Hertwig, many gen- 

 eral and specific objections may be urged. 

 General objections are based upon the con- 

 sideration that the entire cell, cytoplasm as 

 well as nucleus, is concerned in differentia- 

 tion and that neither is capable of embry- 

 onic development in the absence of the 

 other. Differentiation is indeed the result 

 of the interaction of nucleus and cyto- 



plasm, and how then can it be said that 

 the nucleus is the only seat of the inherit- 

 ance material ? If held rigidly, this theory 

 involves the assumption that the cytoplasm 

 and all other parts of the cell are the pro- 

 ducts of the chromosomes, and that there- 

 fore the chromosome and not _the cell is 

 the ultimate independent unit of structure 

 and function— an assumption which is con- 

 trary to fact. Furthermore, since heredity 

 includes a series of fundamental vital 

 processes such as assimilation, growth, 

 division and differentiation, there is some- 

 thing primitive and naive in the view that 

 this most general process can be localized 

 in one specific part of the cell— something 

 which recalls the long-past doctrines that 

 the life was located in the heart or in the 

 blood, or the ancient attempts to find the 

 seat of the soul in the pineal gland or in 

 the ventricles of the brain. 



Among specific objections may be men- 

 tioned the fact that the cytoplasm is not 

 isotropic, as Hertwig supposed, but rather 

 that many fundamental differentiations are 

 found in the cytoplasm of the egg at the 

 time of fertilization and immediately after. 

 As evidences of such differentiations may 

 be cited, (1) polarity and symmetry, (2) 

 differential cleavages, (3) positions and 

 proportions of important organ bases, (4)' 

 various types of Q^g organization, (5) ex- 

 periments in hybridization. 



1. So far as is known, the animal pole 

 of the egg becomes the aboral pole of the 

 gastrula in all animals, while the cytoplasm 

 in this region gives rise to the ectoderm of 

 the developing animal. This polarity of 

 the egg may be traced back through its 

 ovarian history to its earliest stages, and it 

 is probable that in some cases at least it is 

 directly continuous from generation to 

 generation. Here then is an important 

 character which is inherited through the 

 cytoplasm and not through the nucleus. 



2. Similarly, the symmetry of the cyto^ 



