51- PHYSIOLOGICAL MORPHOLOGY 



nucleus and cytoplasm co-operate with each other. The character of 

 the species and the power of continued existence reside neither in the 

 nucleus nor in the cytoplasm, but only in the combination of the two 

 which forms the normal protoplast. Just as a new lichen may be 

 synthesized by changing either the symbiotic fungus or the symbiotic 

 alga, so also might a new species or variety of a plant be produced 

 if the nucleus of one plant could be combined successfully with the 

 cytoplasm of another. The combination of a variety of nuclei with 

 the same cytoplasm, or the combination of one nucleus with different 

 cytoplasms, would in every case originate a specifically distinct organism, 

 for a given protoplast must always contain the same component parts, just 

 as a given lichen does. A change in the external conditions may cause 

 a corresponding, and more or less marked, modification of the shape and 

 character of the organism ; but such change either affects the nucleus 

 and cytoplasm to a relatively equal extent or does not cause the slightest 

 alteration in their inherent nature. 



Until the nuclei and cytoplasms of different plants can be combined 

 together, it is impossible to empirically determine the foregoing problems. 

 Hybrids formed by the union of ent're protoplasts derived from different 

 species exhibit characteristics intermediate between their two parents. Since 

 the sperm l conveys to the ovum not only the male pro-nucleus, but also 

 a certain amount of cytoplasm, it is not justifiable to assume that the 

 nuclei alone are of importance in fertilization. Nor has it been conclusively 

 proved that the hereditary characteristics of the parents are transmitted 

 solely by the nucleus. Boveri 2 found that the non-nucleated ovum of an 

 Echinoderm, when fertilized by the spcrmatozoid from another species, 

 developed the peculiarities which characterized the latter. This is not, 

 however, a conclusive proof that the nucleus alone is concerned with the 

 transmission of acquired characteristics, for the spermatozoon is a protoplast 

 and introduces cytoplasm as well as a nucleus into the ovum. The facts 

 before us by no means justify the conclusion that the nucleus is the centre 

 of all the vital activities of the protoplast, while the cytoplasm is nutritive 

 and entirely subordinate to it. In no case has an isolated mass of cytoplasm 

 been seen to divide spontaneously. Demoor (1. c.) has. however, shown 

 that the nucleus may divide partially or completely after the cytoplasm 

 has been killed, and it is therefore probable that it is the nucleus which 

 initiates cell-division, although in the isolated nucleus complete division 

 may be as impossible as it is in the isolated cytoplasm. 



There is no doubt that the nucleus, formerly comparatively neglected, 



1 On spermatozooids and allied literature, see Belajeff, Flora, Erganzungsband, 1^94, p. i. 

 ' J Boveri, Archiv f. Entwickelungsmechanik, 1895, Bd. II, p. 394; Seeliger. ibid., 1894, Bd. I, 

 p. 204. 



