336 PROCEEDINGS OF THE AMERICAN ACADEMY. 



and when they are about to pass to that part of the body which tliey 

 have to control. This is inchoated by the climatic variations of the 

 butterfly Pulyommatus phlceas." 



He then adds : — 



" The primary cause of variation is always the effect of external 

 influences. Were it possible for growth to take place under abso- 

 lutely constant external influences, variation would not occur; but as 

 this is impossible, all growth is connected with smaller or greater de- 

 viations from the inherited developmental tendency. 



" When these deviations only affect the soma, they give rise to tem- 

 porary non-hereditary variations ; but when they occur in the germ 

 plasm, they are transmitted to the next generation, and cause corre- 

 sponding hereditary variations in the body." (pp. 462, 463.) 



That the physical seat of heredity does exist in the nucleus has 

 been wellnigh demonstrated, if not quite, by some remarkable ex- 

 periments by Boveri at the Naples Zoological Station, so that what 

 was a mere hypothesis has apparently become a matter of fact. 

 Boveri's results appeared in 1889,* and a translation of his short 

 paper has been published by Prof. T. H. Morgan in " The American 

 Naturalist" for March, 1893. 



Five years ago, by accident, the brothers Hertwig discovered that 

 in consequence of shaking, certain eggs of sea-urchins fell to pieces ; 

 some of these pieces contained nuclei and others not. It was found 

 that the non-nucleated pieces could be artificially fertilized as well as 

 those containing nuclei, and that the bits of yolk underwent what is 

 called segmentation. 



Boveri, taking the hint suggested by these happy accidents, made 

 the astonishing discovery that the enucleated bits of eggs could be fer- 

 tilized, and that such bits developed into larvae or young sea-urchins as 

 completely formed as those growing from ordinary entire fertilized eggs. 



The further experiments to prove the seat of heredity were to hy- 

 bridize the fragments of eggs of one genus of sea-urchins with the 

 sperm cells of another genus, and to rear them far enough along in life 

 to determine whether the young showed the qualities of both species 

 or one only. By cross fertilizing the enucleated egg fragments of 

 Sphserechinus with the male germs of Echinus, Boveri produced an 

 almost exact middle form, standing half-way between the two parents. 

 He found however that a portion of the cross-bred larva? agreed entirely 



* Ein gpsclilcchtlidi erzougter Organismus oline miitterliche Eigenschaften. 

 Sitzung der Gesellschaft fur Morphologie und Physiologie zur Miinclien. 

 Sitzung am 16 Juli, 1889. 



