148 THE CAUSES OF SPECIFIC SHAPE 



In tissues protoplasmic transference between neighbouring protoplasts may 

 remove, maintain, or restore the embryonic character of particular cells. Hitherto 

 it has not been found possible to bring isolated embryonic cells or primordia 

 of flowering plants to further development, but the same also applies to the 

 fertilized phanerogamic ovum, which possesses full embryonic powers and would 

 undoubtedly develop if nourished under appropriate external conditions. 



The assumption of certain authors that amitotic nuclear division indicates 

 an incapability for reproduction is at once disproved by the fact that direct 

 nuclear division occurs normally in many typical embryonic cells. Both forms 

 of nuclear division may naturally result in the production of cells incapable of 

 further growth. In Chara the nodal cells divide by karyokinesis and remain 

 capable of reproducing new plants, whereas the internodal cells which divide 

 by amitosis have not this power 1 . The nodes of higher plants often retain high 

 reproductive powers, although these are much restricted in the intact plant in spite 

 of the presence of typical embryonic cells. 



The products of cellular division in plant-primordia at first possess general 

 powers, and subsequently undergo specific differentiation involving reparable or 

 irreparable modifications of these powers. Weismann has assumed the existence 

 of two special protoplasmic masses, one the reproductive, concerned solely in 

 reproduction, the other vegetative or somatic, and carrying out growth and the 

 vegetative functions of the individual. The facts of plant physiology at once 

 reveal the untenable character of this hypothesis 2 . In addition a very large part 

 of the zoological controversies on the development of the ovum has arisen owing 

 to attempts to make the facts fit preconceived ideas, or owing to hasty generaliza- 

 tions from insufficient data 3 . If all these theories and the special terms connected 

 with them are put on one side, a general agreement exists between the facts ob- 

 served in plants and in animals, for in both cases the specific differentiation of the 

 embryonic cells of an organism is determined by their position and by the general 

 external conditions brought to bear upon them. The observed facts have, however, 

 received various interpretations, and it is also disputed whether cells, which in the 

 intact organism develop always in the same manner, are specifically differentiated 

 at a comparatively early period of development or whether they retain general 

 embryonic powers for a long time. In this respect neighbouring cells may behave 

 very differently, so that the dissimilarity is merely one of degree, and is due to 

 the fact that the changes in question occur at earlier or later dates. Plants afford 

 much better material than animals for the study of such problems, for, owing to 

 the high specialization of the individual cells of animals, they are far less capable of 

 reproducing the entire organism than are the less highly differentiated ones of plants. 



1 Debski, Jahrb. f. wiss. Bot., 1898, Bd. xxxn, p. 656; Hegler, Bot. Centralbl., 1895, 

 Bd. LXIV, p. 203; Strasburger, Histologische Beitrage, 1893, Heft 5, p. 99; Richter, Flora, 

 1894, p. 417. 



3 O. Hertwig, Die Zelle u. d. Gewebe, 1898, II, p. 58, and the literature there given. 



3 Cf. O. Hertwig, 1. c., pp. 58, 212 ; Zeit- und Streitfragen d. Biologic, 1897, Heft 2 ; Roux, 

 Programm u. Forschungsmethoden d. Entwickelungsmechanik, 1897; Driesch, Analyt. Theorie d. 

 organ. Entwickelung, 1894; Resultate u. Probleme d. Entwickelungsphysiol. d. Thiere, 1898 (Ergeb. 

 d. Anat. u. Entwickelungsgesch., edited by Merkel and Bonnet, Bd. vm). 



