54 Thomas HcMiry Huxley 



placenta by which the einbr}-© of a mammal receives 

 nourishment from the blood of the mother. These 

 embr3^os grow up into the solitary form, and the soli- 

 tary form gives rise to a long chain of the aggregate 

 form which developes in the interior of the body, 

 Chamisso compared this progress to the development 

 of insects. " Supposing," he said, " caterpillars did 

 not bodily change into butterflies, but by a process of 

 sexual breeding produced young which grew into the 

 ordinary adults, and that these adults, as indeed thej^ 

 do, gave rise to caterpillars by sexual reproduction, 

 then there would be a true alternation of generations." 

 The first generation would give rise to a second gene- 

 ration totally unlike itself, and this second generation 

 would reproduce, not its kind, but the first generation ; 

 such an alternation of generations he stated to occur 

 among the salps. Huxley had an excellent opportun- 

 ity to study this question at Cape York in November, 

 1849. " For a time the sea was absolutely crowded 

 with Salpse, in all stages of growth, and of size very 

 convenient for examination." He was able to verify 

 the general truth of Chamisso' s statement. The aggre- 

 gate form of Salpa always gives rise to the solitary 

 salps, and the solitary salps always give rise to chains 

 of the aggregate salps. But the process of reproduc- 

 tion he shewed to be quite different in the two cases. 

 The solitary salp produces in its interior a little stolon 

 or diverticulum which contains an outgrowth from the 

 circulato^-y system, and this stolon gradually becomes 

 pinched off into the members of the chain of the aggre- 

 gate form. The salps of the aggregate form are there- 

 fore merely buds from the solitary form, and are not 

 produced in the ordinary way, by sexual generation. 

 On the other hand, each salp of the chain has within it 



