ON FORMATIVE INDUCTION AND PECULIARITIES OF CELLS 147 



non-vital stimuli. It is of course possible that the ova of highly specialized 

 plants lack a portion of the germ-plasma which the sperm adds to them, 

 but it is equally possible that in many cases at least they may be capable 

 of development when appropriately stimulated. 



In any case the possibility that fertilization involves a stimulatory 

 reaction as well as the addition of the missing germ-plasma must be borne 

 in mind, and it is in fact possible that the two reactions may be separated 

 and that a certain development may be induced in the ovum without the 

 introduction of any living material. Winkler in fact found that a watery 

 extract of the sperm of an Echi-noderm induced partial segmentation of the 

 ovum, and Loeb states that treatment with a solution of magnesium chloride, 

 or even a temporary increase in the osmotic pressure of the surrounding 

 medium, produced the same effect 1 . 



Nathansohn found that the ovum of Marsika vestita at i8C. only produces 

 an embryo when fertilized, whereas at 35 C. a certain number of the ova develop 

 parthenogenetically. Most of the ova of Marsika Drummondii do the same at 

 i8C., whereas at 9C. only a few are capable of parthenogenetic development. 

 A rise of temperature therefore favours parthenogenesis in both cases, and the 

 previous cultural conditions, as well as the ripeness of the original spores, seem 

 also to exercise a certain influence. 



The induction and inhibition of growth in both vegetative and reproductive 

 cells are not always the result of chemical stimuli, and the latter, though the 

 character of the substance is of importance, do not always depend upon 

 dissociation, i. e. upon the number and properties of the ions in solution. Fertiliza- 

 tion is certainly not a mere process of chemical stimulation, as Loeb supposes, 

 for the union of two protoplasts to form a new organism involves a mixing of 

 characters which may result in the appearance of new peculiarities capable of 

 hereditary transmission. 



Since the fact that a cell possesses full embryonic reproductive powers 

 is only shown when the conditions permit or induce the exercise of these 

 powers, we cannot always with safety say which cells in an inactive tissue 

 retain their embryonic powers, and which have lost them. The former seems 

 to apply to all the living cells of mosses, and to certain of those of flower- 

 ing-plants, such as the cambial, pericyclic, and cortical cells. It may often 

 be the case that an embryonic protoplast may prevent any further exercise 

 of its reproductive powers by enclosing itself in a cell-wall incapable of 

 further growth. 



1 Winkler, Ueber die Furchnng unbefrnchteter Eier : Nachr. d. Ges. d. Wiss. z. Gottingen, 

 1900, Heft 2 ; Loeb, American Journal of Physiology, 1899, Vol. Ill, pp. 137, 434 ; 1900, Vol. IV, 

 pp. 178, 430. 



2 Nathansohn, Ber. d. Bot. Ges., 1900, p. 99. The literature on parthenogenesis in plants 

 is given there. Cf. also Klebs, Jahrb. f. wiss. Bot., 1900, Bd. xxxv, p. 189. 



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