28 



LECTURE II. 



Nassula. 



cella and some other species, we have examples of spontaneous divi- 

 sion in the longitudinal 

 direction, which com- 

 mences at the mouth {Jig. 

 5. b.), and extends {ib. 

 c, d.), to the irritable and 

 contractile stem, from 

 which one or both of the 

 new-formed individuals 

 detach themselves (ib. e, 

 /.). Both the longitudinal 

 and transverse mode of 

 fission may take place in the same species (as in the Glaucoma, 

 figured in Jiff. 6, 3 — 8.), and as in the Chilodon cuculluliis above 

 cited. Cohn* observed, in Loxodes Bursaria, that the two indivi- 

 duals produced by longitudinal fission had almost the same size and 

 shape as the previous single individual ; whilst the two resulting 

 from transverse fission seemed longer to remain as mere halves or 

 mutilated individuals. Sometimes one of the individuals or halves 

 from the longitudinal bisection will set up another act of fission be- 

 fore it has quite separated from its fellow. The circulation of the 

 green globules is arrested during the process of spontaneous fission. | 

 In some species this spontaneous fission, which corresj)onds in so 

 interesting a manner with the earliest phenomenon in the develop- 

 ment of the ovum in the higher animals f , is arrested before its com- 

 15 pletion, but the partially separated individuals 

 continue in organic connection and form 

 compound animals, sometimes in the form of 

 long chains, sometimes branched, sometimes 

 expanding to form a spherical bag, as in the 

 well-known Volvox globator, which was long 

 deemed a single individual of a peculiar 

 species. New spherical groups of Volvoces 



* XVIII. 



f Beccaria, who first saAV a Polygastrian in the act of dividing, supposed it to be 

 two in copulation. Saussure, in 1765, first recognised its real character. 



J This analogy I pointed ont in ilhistration of the cleavage -process of the ova 

 of the Mednsa in my "Lectures on Generation," delivered in 1840. It is alhided 

 to in the following passage in Dr. ISI. Barry's " Memoir on the Nucleus of the Animal 

 and Vegetable Cell," 8vo. 1847. "Between the appearances presented by the 

 mammiferous germ during the passage of the ovum through the oviduct, and cer- 

 tain infusoria, including the Volvox globator as figured by Ehrenberg, the resem- 

 blance first mentior^ed by Professor Owen is so remarkable that we cannot avoid 

 the beUef, that the same process operates in both." 



