234 LECTURE XVIII. 



viz. that the generation of the Aphides took place, as in some En- 

 tozoa, by the individualisation of a previously organised tissue.* No 

 one, however, has observed a portion of mucous membrane, muscular 

 or nervous fibre, or other organised tissue detach and transform itself 

 into an Entozoon ; such a process is as hypothetical and as little in 

 accordance with observed phenomena as spontaneous generation. The 

 fissiparous nucleated cells, once metamorphosed into a tissvie, can pro- 

 duce notliing further; but those which retain their primitive state amidst 

 the various tissues which the rest have constituted in building up the 

 body of the new animal, may, by virtue of their fissiparous and assi- 

 milative forces, produce something further. They may give rise to a 

 sviccession of similar cells which may float, as blood-discs, in the cir- 

 culating stream, to be afterwards converted into the different tissues 

 of the individual ; they may, as in the Aphides, retain sufficient of 

 the fecundative and organising forces to disseminate the like virtue 

 through their multiplied subdivisions, and give rise to the different 

 tissues and organs of a new individual, which in its turn may include 

 some unmetamorphosed nucleated cells, with organising energies si- 

 milar to those of the parent cell of which they were the fissiparous 

 progeny. 



Tlie individual Aphides thus generated are all, until the last 

 brood, females, which are brought forth as larva, and generate and 

 perish under that form. The last brood includes both sexes, 

 which acquire their full development and winged state, and the 

 females exclude impregnated ova. The gemmiparous procreation 

 by the larval polype of the Medusa of similar larvaa ; the subsequent 

 acquisition by both of a more concentrated form, generating the 

 young discoid Medusae by a series of spontaneous divisions ; lastly, 

 the acquisition of the distinct sexes and the procreation by impreg- 

 nated ova by the perfect Medusae, are phenomena essentially ana- 

 logous to those of Aphidian generation. 



The larvae brought forth by the apterous and virgin Aphis have 

 reached a more advanced state of development than the normally 

 generated larvae of the Coleoptera and Lepidoptera. The compound 

 eyes are developed on the head, and the antennae have acquired 

 their mature form and proportions : the six thoracic legs have at- 

 tained their due length, are divided into the normal number of 

 joints, and gain the requisite firmness for use almost immediately 



* " A dire vrai, je me refuse ii cmettre une opinion au milieu d'lm tel tledale, 

 ot je tiens pour plus ])hilosophique d'avouer son ignorance dans un phenoinene ou 

 la nature nous refuse meme I'apparence d'une explication. S'il fallait une expli- 

 cation a toute force, j'admettrais que la generation se fait ici, comme ehez quelques 

 Entozoaires, par individualisation d'uii tissu preccdemment organise." 



