366 METAMORPHOSIS 



occurrence of reproductive organs we must admit that we have as yet arrived 

 at no clear appreciation of the factors which determine the existence of several 

 types of these organs. There are many organisms which persist and multiply 

 exclusively by one mode of propagation. As above observed, MOBIUS recognizes 

 buds and germs, and distinguishes them by their mode of origin. Germs arise 

 by rejuvenescence, buds by growth and ordinary cell division. Rejuvenescence 

 is illustrated in the formation of the ovum, as also in the development of 

 swarmspores. It is not quite so obvious, however, why the origin of a new mem- 

 brane a phenomenon which did not occur in Oedogonium should be of so 

 much importance to the organism, since the old membrane was just as capable 

 of growth as the new one in this case. But in describing the phenomenon as 

 ' rejuvenescence ' we are regarding not so much the cell-wall as the internal 

 rejuvenescence of the protoplasm. STRASBURGER (1880) has shown that the 

 protoplasm undergoes a reconstruction when swarmspores are formed in Oedo- 

 gonium ; the nucleus moves through the green protoplasm towards the outside, 

 and later on resumes its old position. BERTHOLD (1886, p. 289) has observed 

 similar rearrangements in other cases of cell-formation. It is not clear, how- 

 ever, how such processes bring about a rejuvenescence, and one idea underlies 

 all conceptions of this nature, viz. that protoplasm degenerates, gets worn out 

 by perpetual growth and cell division, and this conception is one which we have 

 striven to oppose. If protoplasm wears itself out, it has also no doubt the 

 power of regenerating itself. 



There is one point which we have left undiscussed in our consideration of 

 these questions, viz. why must the formation of many resting spores in the Thai- 

 lophyta be preceded by a fusion of cells, a phenomenon which may in the ex- 

 treme case be regarded as a sexual act ? Why should the ovum in the Pterido- 

 phyta and the Phanerogams develop only after being fertilized by the spermato- 

 zoid ? We shall consider this question in the next lecture. 



Bibliography to Lecture XXVIII. 



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[BOWER. 1887. Trans. Linn. Soc.] 



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STRASBURGER. 1880. Zellbildung u. Zellteilung. 3rd ed., p. 81. 

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