REPRODUCTION AND SEX 481 



other Hymenoptera, such as ants, {d) The term seasonal partheno- 

 genesis may be applied to cases like greenflies or Aphides, where 

 one parthenogenetic generation succeeds another all through the 

 summer, but males reappear in the autumn and fertilisation occurs. 

 This is also illustrated by some of the water-fleas, {e) The term 

 juvenile parthenogenesis may be applied to some curious cases 

 (e.g. in the midge Miastor) where larval forms exhibit precocious 

 reproductivity without any fertilisation. It becomes difhcult, how- 

 ever, to draw a line between such cases and multiplication by means 

 of spores, such as is seen in the larval stages of the liver-fluke and 

 in many plants. Spores are specialised reproductive cells which 

 develop without fertilisation; they are familiar to everyone on the 

 fronds of ferns. The formation of spores is a primitive mode 

 of reproduction, but the parthenogenetic development of ova 

 is probably in all cases secondary and derivative — a relapse 

 from the normal spermic development. None the less it seems to 

 work well in certain kinds of organisms and in certain conditions 

 of life. 



It may be asked whether egg-cells which normally develop 

 without being fertilised are in any way different from ordinary ova. 

 But the answer is not at present very clear. In some cases (ants, 

 bees, and wasps) the ova go through the ordinary process of matura- 

 tion, involving a reduction of the number of nuclear rods or chro- 

 mosomes to half the normal number. In some other cases (Rotifers, 

 some water-fleas, and greenflies) there is no reduction when the 

 conditions of life are favourable, though there may be when they 

 are unpropitious. 



ARTIFICIAL PARTHENOGENESIS.—Of interest is the fact that 

 in a variety of cases the ovum may be artificially induced to 

 develop parthenogenetically. The demonstration of this has been 

 mainly due to Jacques Loeb and Yves Delage. If the unfertilised 

 eggs of a sea-urchin be left for a couple of hours in sea-water, the 

 composition of which has been altered (e.g. by adding magnesium 

 chloride), and be then restored to ordinary sea- water, many of them 

 develop into normal larvae. A mixture that Delage found to be very 

 effective for sea-urchin ova consisted of 300 c.c. of sea-water, 

 700 c.c. of an isotonic solution of saccharose, 15 centigrams of 

 tannin dissolved in distilled water, and 3 c.c. of normal ammo- 

 niacal solution. It works equally well if the volume of the sea- 

 water or of the saccharose be doubled. The ova were left for an hour 

 in the mixture, then washed several times, and then placed in sea- 

 water, where they soon developed. In a few cases fully-formed sea- 

 urchins have been reared. There are two points of special importance : 

 first, that the artificial parthenogenesis has been induced in a great 

 variety of types, e.g. sea-urchin, starfish, marine worm, mollusc, 



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