PHYSIOLOGICAL RELATIONS OF NUCLEUS AND CYTOPLASM 



every part of the latter may show the characteristics of either or both 

 parents. 



Boveri ('89, '95, i) has attempted to test this conclusion by a most 

 ingenious and beautiful experiment ; and although his conclusions do 

 not rest on absolutely certain ground, they at least open the way to 

 a decisive test. The Hertwig brothers showed that the eggs of sea- 

 urchins might be broken into pieces by shaking, and that spermatozoa 

 would enter the enucleated fragments and cause them to segment. 

 Boveri proved that such a fragment, if fertilized by a spermatozoon, 

 would even give rise to a dwarf larva, indistinguishable from the nor- 

 mal in general appearance except in size. The nuclei of such larvae 

 are considerably smaller than those of the normal larvae, and were 

 shown by Morgan ('95, 4) to contain only half the number of chromo- 

 somes, thus demonstrating their origin from a single sperm-nucleus. 

 Now, by fertilizing enucleated egg-fragments of one species (Sphce- 

 rechinus grannlaris) with the spermatozoa of another (Echinus micro- 

 tuberculatus)) Boveri obtained in a few instances dwarf Plutei show- 

 ing except in size the pure paternal characters (i.e. those of Echinus, 

 Fig. 164). From this he concluded that the maternal cytoplasm has 

 no determining effect on the offspring, but supplies only the material 

 in which the sperm-nucleus operates. Inheritance is, therefore, ef- 

 fected by the nucleus alone. 



The later studies of Seeliger ('94), Morgan ('95, 4), and Drisch 

 ('98, 3) showed that this result is not entirely conclusive, since hybrid 

 larvae arising by the fertilization of an entire ovum of one species by 

 a spermatozoon of the other show a very considerable range of varia- 

 tion ; and while most such hybrids are intermediate in character 

 between the two species, some individuals may nearly approximate 

 to the characters of the father or the mother. Despite this fact 

 Boveri ('95, i) has strongly defended his conclusion, though admitting 

 that only further research can definitely decide the question. It is 

 to be hoped that this highly ingenious experiment may be repeated 

 on other forms which may afford a decisive result 



5. The Nucleus in Maturation 



Scarcely less convincing, finally, is the contrast between nucleus 

 and cytoplasm in the maturation of the germ-cells. It is scarcely 

 an exaggeration to say that the whole process of maturation, in its 

 broadest sense, renders the cytoplasm of the germ-cells as unlike, 

 the nuclei as like, as possible. The latter undergo a series of com- 

 plicated changes which result in a perfect equivalence between them 

 at the time of their union, and, more remotely, a perfect equality of 

 distribution to the embryonic cells. The cytoplasm, on the other 



2 A 



