t 



AND MORPHOLOGY OF APHIS. 217 



chioj)oda. It is not known to occur in any of the Lamellibranchiata; and among the 

 higlicr Ilollusca the nearest approach to it is presented by the animal (whatever it is) 

 which gives rise to the " Synapta-schnecken" (high Gasteropods), and by the Hectocoty- 

 ligcnous Cephalopoda. 



After this simple statement of well-knomi facts, I need not remind e^'cn the tyro in 

 zoology, that there is no evidence of an inverse relation between the occurrence of aga- 

 mogenesis and complexity of organization. 



I have hitherto, in the com-sc of this argument, confined myself in the main to the deve- 

 lopment of Aphis ; but it is only just to observe that the author of the hypothesis brings 

 forward yet another original observation in support of his large generalization : — 



" In the freshwater polype, the progeny of the primary impregnated germ-cell retained 

 unaltered in that body, may set up, under favoiirable stimuli of light, heat, and nutriment, 

 the same actions as those to which they owed their own origin ; certain of the nucleated cells 

 do set up such actions, those, e.g. in the Hydra fusca, which are aggregated near the adhe- 

 ring pedicle or foot ; and the result of their increase by assimilation and multiplication 

 is, to push out the contiguous integument in the form of a bud, which becomes the seat 

 of the subsequent processes of growth and development ; a clear cavity or centre of assi- 

 milation is first formed, which soon opens into the stomach of the parent ; but the com- 

 munication is afterwards closed, and the young hydra is ultimately cast off from the surface 

 of the parent*." — ' Lectm-es,' 2nd ed. p. 124. 



I have had occasion carefully to watch the process of gemmation not only in Hydra, 

 but in many species of aU the other subdivisions of the Hydrozoa ; and I venture to assert 

 that no such process as that described by Prof. Owen takes place in any one of them. 



The bud is from the fii'St in commimication with the cavity of the body, of which it is 

 a mere diverticulum, whose walls are a little thickened at the extremity. No special cell 

 or group of cells can be discovered as the centres whence growth proceeds. No " inte- 

 gument" is pushed out by any thing beneath it; but the outer layer of the body of 

 the animal thickens and grows pari passu with the growth of the bud. No especial 

 accumulation of derivative germ-cells can be seen in any part of the body of any Uydro- 

 zooii ; and before gemmation commences there is no distinguishable difference of texture 

 between the part in which gemmation commences and any other portion of the body. 

 Furthermore if a complex Hydrozoon, such as a Fhysophora or Agalma, be examined, it 

 will be found that there is no histological distinction whatsoever between that part of the 

 body which is to give rise to a free swimming generative zooid, and that which produces 

 merely a bract, a tentacle, or a stomach. 



In this case then, as in that of the Apjhis, the hypothesis receives no support from, but 

 is totally opposed by, facts ; and I unreservedly adopt the conclusion (long since clearly and 

 well expressed by Dr. Carpenter), that " spermatic force " is but a name without definite 

 meaning, applied to that which is not proven to exist, and the assumption of whose 

 existence, even, does not help us a single step towards the understanding of the wonderful 

 phenomena of agamogenesis. 



* I have cited this passage from the ' Lectures ' rather than from tlie work on " Parthenogenesis," as they may 

 be supposed to contain the expression of the author's latest riews. 



