FOUNDATION OF A THEORY OF HEREDITY. 187 



for we have only to transfer the primary cause of development, from 

 an unknown source within the organism, into the nuclear sub- 

 stance, in order to make the views identical. 



It appears at first sight that the knowledge which has been 

 gained by studying the indirect division of nuclei is opposed to 

 such a view, for we know that each mother-loop of the so-called 

 nuclear plate divides longitudinally into two exactly equal halves, 

 which can be stained and thus rendered visible. 



In this way each resulting daughter-nucleus receives an equal 

 supply of halves, and it therefore appears that the two nuclei must 

 be completely identical. This at least is Strasburger's conclusion, 

 and he regards such identity as a fundamental fact, which cannot 

 be shaken, and with which all attempts at further explanation must 

 be brought into accord. 



How then can the gradual transformation of the nuclear substance 

 be brought about ? For such a transformation must necessarily 

 take place if the nuclear substance is really the determining factor 

 in development. Strasburger attempts to support his hypothesis 

 by assuming that the inequality of the daughter-nuclei arises from 

 unequal nutrition ; and he therefore considers that the inequality 

 is brought about after the division of the nucleus and of the cell. 

 Strasburger has shown, in a manner which is above all criticism, 

 that the nucleus derives its nutrition from the cell-body, but then 

 the cell-bodies of the two ex hypotliesi identical daughter-nuclei 

 must be different from the first, if they are to influence their nuclei 

 in different ways. But if the nucleus determines the nature of 

 the cell, it follows that two identical daughter-nuclei which have 

 arisen by division within one mother-cell cannot come to possess 

 unequal cell-bodies. As a matter of fact, however, the cell-bodies of 

 two daughter-cells often differ in size, in appearance, and in their 

 subsequent history, and these facts are sufficient to prove that in 

 such cases the division of the nucleus must have been unequal. 

 It appears to me to be a necessary conclusion that, in such an in- 

 stance, the mother-nucleus must have been capable of splitting into 

 nuclear substances of differing quality. I think that, in his argu- 

 ment, Strasburger has over-estimated the support afforded by exact 

 observations upon indirect nuclear division. Certainly the fact, 

 discovered by Flemming, and more exactly studied by Balbiani and 

 Pfitzner, that, in nuclear division, the loops split longitudinally, is 



