14 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
Some Problems of Reproduction.* — Prof. M. M. Hartog discusses 
some questions of reproduction by the light of modern inquiries. He 
points out that the only absolutely agamous forms are to be found 
among the Monadinese, where rest is the only agent of rejuvenescence. 
A frequent mode of rejuvenescence is a change in the mode of life ; this 
is seen in apogamous and self-fertilizing organisms. In the higher 
Monadineae and the Myxomycetes a plasmodium is formed, and the 
cytoplasm is renewed by plastogamy, while the nuclei wander from their 
original cytoplasts. Isogamy is the next advance, and it involves karyo- 
gamy or the reconstitution of a nucleus by the fusion of old ones. Here 
rejuvenescence takes the form of a new cell association. A similar 
rejuvenescence may take place by the mere migration of a nucleus into a 
vacant foreign cytoplast, as in the union of a spermatozoon with the non- 
nucleated fragment of the egg of an Echinoderm. Organisms that have 
attained the capability of karyogamic rejuvenescence may, by prolonged 
fissile reproduction without karyogamy, pass into a sessile condition 
marked by reproductive incapacity ; here, therefore, karyogamic reju- 
venescence has become essential to the preservation of the race. 
Rapidly repeated nuclear fissions, without sufficient interval for 
nutrition and recovery, may lower the vital energy or constitution of the 
cell, and accelerate this reproductive incapacity : this may be the physio- 
logical import of the fissions that so frequently differentiate the gamete 
and determine its obligatory character. However, the reproductive 
incapacity of most microgametes is sufficiently explained by the extreme 
reduction of their cytoplasm. The incapacity due to long or repeated 
acts of fission uninterrupted by karyogamy is a matter of constitutional 
temperament or vigour characteristic only of the race, for 
(a) It is absent in primitive, agamous types, 
( b ) It is slight in groups where parthenogenesis occurs, though often 
obsolete in closely-allied forms, 
(c) It has been lost in apogamous forms. 
A further evolution of this constitutional weakness takes place in 
forms which are either exogamous or sexually differentiated ; here the 
nuclei that fuse to remove this reproductive incapacity by rejuvenescence 
must be of distinct origin. Exogamy of isogametes cannot be taken as 
indicating latent sex, for it is merely the expression of karyogamic in- 
compatibility of close blood-relations ; which, under the name of allo- 
gamy, has been long since recognized when associated with and super- 
added to bisexuality. 
The constitutional weakness reaches its highest degree in those 
organisms where allogamy is most marked. In all phases of plasmodial 
aud karyogamic rejuvenescence we find the migration of the nucleus to 
foreign cytoplasm or the reconstitution of the cytoplasm or of the 
nucleus, or a combination of them, to be the sole necessary factors, and we 
infer, therefore, that the constitutional weakness of the later terms of a 
cycle of fission is largely due to the continuance of the association of 
nucleus aud cytoplast unchanged. The author suggests that the evil 
effects of the prolonged association of cell and nucleus are due (a) to the 
nucleus responding less actively to the stimuli from the cytoplasm ; (b) 
its consequently inadequate directive power ; (c) to the resulting bad 
* Quart. Journ. Micr. Sci., xxxiii. (1891) pp. 1-79. 
