32 BIOLOGICAL LECTURES. 



pangcncs of De Vries, the plasonics of Wiesner, which assump- 

 tion, in one form or another, seejns to me a necessity), the 

 independent unit of structure is still the entire cell, not cyto- 

 plasm alone, nor nucleus alone, but the two together. So far 

 as we know, the cytoplasm of every cell comes from the cyto- 

 plasm of some parental cell, just as certainly as the nucleus 

 comes from a preceding nucleus. Until, therefore, it can be 

 shown that the nucleus can exist independently of the cyto- 

 plasm, it is unsound to assert that the cytoplasm is not an 

 essential constituent of the cell. And until some one has 

 shown that cytoplasm is not derived from pre-existing cyto- 

 plasm, but is the product of the nucleus,^ it is too soon 

 to assert that the control of the entire cell lies in the 

 nucleus, and hence that the nucleus is the sole bearer of 

 heredity. 



In seeking to show that the nucleus is not the sole bearer 

 of heredity, we are not confined entirely to the a priori argu- 

 ment ; aside from the important evidence already adduced in 

 the presence and function of the asters observations are not 

 altogether wanting to show that the cytoplasm, in many 

 respects at least, is not controlled b}' the nucleus. In the 

 early cleavage stages of Crepidnla plana, it can be shown beyond 

 question that the direction of the cleavage, the size of the cells 

 formed, and the shape of those cells is the result of cytoplasmic 

 activity rather than of nuclear.'-^ It is generally believed that 



^ This is, I know, what Weismann asserts (The Germ I'lasm, p. 50), and he 

 urges in proof the observations of Riickert on the alteration in size of the 

 chromosomes of the nucleus during the growth of the ovum ot the dogfish. 

 These chromosomes first enlarge greatly, and afterward diminish in size until 

 they are not much larger than at first. Riickert, therefore, believes that the 

 chromosomes give off a large amount of substance to the cytoplasm, and this 

 conclusion is probably correct. It does not follow, however, that this material 

 controls the cytoplasm, as Weismann assumes, any more than the fact that the 

 cyto|)lasm supj^lies the nucleus with a large amount of nuclear sap in its j^re- 

 division stages, argues that the cytoplasm controls the nucleus. It is probable 

 that each supplies substances to the other, and it is interesting to note that at 

 the very stage described by Riickert (the pre-division stage), in which the chro- 

 mosomes are giving off substances to the cytoplasm, the nucleus, in many 

 forms at least, is receiving a large amount of material in the form of nuclear 

 sap from the cytoplasm. 



2 Since this lecture was given, I have found that Boveri, in his recent work on 



