AND THE LOWER ANIMALS. 207 



sown them; as one would corn, and that half the 

 arguments evoked in support of the doctrine of spon- 

 taneous generation are therefore annihilated. 



There remain the arguments, borrowed from the 

 history of the Helminthes, especially from the cir- 

 cumstance of the isolation of certain species, from the 

 absence in them of a reproductive apparatus, and 

 from their existence in the closed cavities and interior 

 of the tissues. But are these arguments better based 

 than the others ? Are certain Helminthes, if not all 

 — by an exception henceforth to be regarded as 

 special — developed spontaneously in those localities 

 in which the scalpel encounters them ? 



It is to embryogeny alone that we can look for a 

 reply to this question, and efforts have been made for 

 several years past to solve this latter difficulty. 



Numerous and important, though isolated dis- 

 coveries have been made, in France by F. Dujardin, and 

 in Germany by Bojanus, Baer, Kolliker, Nordmann, 

 Siebold, Wagner, and others. No intestinal worm had 

 been observed in the early stages of its evolution. 

 One constantly came in contact with agamic species ; 

 and it was so hard to explain their existence, that, 

 for twenty years, even the least cautious naturalists 

 admitted that here there must be a series of meta- 

 morphoses resembling that of insects.* Such was 

 the state of science in 1840. We knew absolutely 

 nothing of the embryogeny of either Cestoid or Cystic 

 worms. 



Concerning the Trematodes, it was said that in the 



* Siebold's observations on Monostomum mutabilc were made in 

 1835. They prepared the way for a series of discoveries, now 

 very considerable and constantly increasing. 



