SOME PROBLEMS OF REPRODUCTION. 69 
as in karyogamy. And this very feat has been accomplished ; 
for the Hertwigs some years ago showed that Echinoderm 
eggs when shaken up in sea water break into fragments ; and 
observed a spermatozoon entering such a non-nucleated frag- 
ment and segmenting therein, like the zygote segmentation- 
nucleus. Since then, as O. Hertwig recalls in the above-cited 
paper,! Boveri has repeated the observations, and proved that 
the normal morula was formed and developed into a larva. I 
do not see how this is consistent with any theory of karyogamy 
but Biitschli’s—that it is a process of rejuvenescence, to which 
term we are now endeavouring to attach adefinite connotation. 
Our definition of rejuvenescence, karyogamic or other, 
is that it is essentially a process of constitutional 
invigoration,® as its converse, senescence, is one of consti- 
tutional enfeeblement. We can now, grasping this idea, under- 
stand the continued existence of agamous and apogamous forms 
side by side with those where not merely karyogamy, but allo- 
gamous sexual reproduction is essential. Every arrangement 
that makes for protection and comfort tends to become by habit 
indispensable, and the privation of such an “‘ acquired need” 
may produce effects none the less disastrous because it was ac- 
quired, and not primitive. A couple of examples from human 
life will illustrate this. ‘The Maoris found scant clothing neces- 
sary in their cool but not extreme climate until the Europeans 
introduced blankets ; but now their occasional reversion to the 
practice of going unclothed is said to lead to disastrous results. 
Civilised nations who cook their food largely escape the attacks 
of entozoa: but, on the other hand, when they do occur, 
these attacks disturb and disorder the system the more 
seriously for their rarity; while the Abyssinian, who feeds 
daily on raw beef, thinks it positively unlucky to be without 
a tapeworm in his intestines. The coexistence of agamous, 
karyogamous, aud apogamous types proves that the need for 
karyogamy belongs to the class of acquired needs or necessary 
1 § Vergleich der Hi,’ &., p. 85. I have not been able to consult Boveri's 
original paper in the libraries of our scientific societies. 
2 Of course I use ‘‘ constitutional” in the medical sense. 
