REPRODUCTION AND HEREDITY 263 



afterwards the proglottides begin to sprout as soon as the 

 heads have reached a suitable animal-host. The fertility 

 particular of T. echinococcus is enormous. The sex-form, the 

 'worm ' proper, lives in the intestines of the domestic dog. Its 

 length rarely exceeds -J- centimetre, but it is all the more dan- 

 gerous for being so easily overlooked. If many lovers of dogs 

 knew to what dangers they expose themselves and their family 

 by petting their dogs or even kissing them they would exercise 

 greater caution, for if mature proglottides or ova of Echinococcus 

 reach the intestinal canal of man they rapidly develop into 

 embryos which penetrate the tissues and settle in the brain, lung, 

 or other organs. What terrible pains they are able to cause 

 will be understood when we hear that these cysticerci often 

 reach the size of a human head and a weight of 20 Ibs. On the 

 large original cysticercus-bladder are produced by invagination 

 many thousands of smaller bladders, and from each of these 

 vesicles originate the minute cysts in which the tapeworm-heads 

 are ' rudimentary.' We observe, therefore, a regular alternation 

 between a sexual generation, the tapeworm, and an asexual, the 

 cysticercus, which reproduces by budding. In this case it was 

 probably the extremely favourable conditions of nutrition which 

 gradually induced the larvae to proceed to asexual reproduction. 



Those who have kept a fresh- water-aquarium are well 

 acquainted with the agile water-flea, Daphnia. In tine weather 

 our ditches, ponds, and lakes are often so peopled with them 

 that it is only necessary to draw a fine net through the water 

 to capture numberless thousands. Many observers will have 

 noticed that all water-fleas which are caught in the sea during 

 summer are females. No matter how much we may search, it 

 seems impossible to discover a male. Nevertheless, the animals 

 are not doomed to sterility, for they are able to produce numerous 

 young and these can proceed to reproduction in their turn, but 

 still no male water-fleas are to be found. To take these tiny 

 crustaceans for hermaphrodites able to produce male spermatozoa 

 in addition to ova would be wrong, for they are typical females. 



After reproduction has in this manner proceeded through 

 numerous generations until the approach of the autumn, all at 

 once male individuals appear, easily noticeable to the trained 

 eye by their smaller size. The females again produce ova, and 



