218 PROF. HUXLEY ON THE AGAMIC REPRODUCTION 



Truly we may say, with Degeer {I. c. p. 129), " Les Pucerons sont des insectes bien 

 capables de d^i'anger tout systfeme form^ de geiK^ration, et de mettre en d^route tovis ceux 

 qiii s'elforcent d'expliqvier ce mystfere de la natiire." 



But the question may be asked : if the " spermatic force" be a myth, what is the cause 

 of the phenomena ? Considering that the groundwork of modern physiology is not a 

 score of years old, I do not think the confession of our inability to answer that question 

 at present is any opprobrium to science. 



When we know why, in a mass of tissue of identical structure throughout, one part 

 becomes a brain, and another a heart, and a thii'd a liver — when we can answer these 

 every-day questions of the sphinx, we may attempt her more difficult riddles without 

 running too great a risk of being dcA'oured. 



At the present time it seems to me well nigh hopeless to look for an explanation of 

 these phenomena. Some such classification of them, however, as will indicate their 

 analogies with other vital manifestations, may fairly be attempted, and, when successfully 

 carried out, will prove the fii'st step towards an explanation. 



§ 7. Classification of the Phenomena of Agamogenesis. 



It does not seem to be very difficult to eifect such a classification. In the course of the 

 development of the total product of a single impregnated ovum (which, with Dr. Carpenter, 

 I regard as the zoological indi\'idual), one of two things may occur : either all the living 

 products may remain in connexion with one another, or they may become separated from 

 one another. The former case I term Continuous, the latter Discontinuous Development. 



In continuous development, the size may increase, the form and texture remaining 

 unchanged — constituting simple growth ; or, the size remaining unchanged, the form 

 and texture may alter — constituting simple metamorphosis ; or the two processes may be 

 combined, as in all those changes which we term gemmation, without separation from 

 the parent. 



Discontmuous development differs from continvious only in this, that the products of 

 the growth and metamorphosis of the emljryo become separated into two or more portions, 

 which when they retain their vitality independently are termed " zooids." 



When the produced " zooid " is capable of development into an independent organism 

 mthout the influence of an act of conjugation with another zooid, I term the process 

 agamogenesis. The producing zooid may be devoid of sexual organs, as in the Salpce, 

 many Hydrozoa, many Trematoda — in fact, in the great majority of cases of agamogenesis. 



I term the fii'st producing zooid of the individual the protozooid ; the produced zooids, 

 deuterozooids. In some cases the deuterozooids acqviu-e sexual organs, and give rise to 

 ova and spermatozoa ; but in others they produce new zooids : thus broods of tritozooids, 

 &c., will be produced. When the producing or protozooid possesses no sexual organs, 

 I think Prof. Owen's term of " metagenesis " might well be applied to the kind of 

 agamogenesis ; but where the j^rotozooid possesses sexual organs, and its buds have all 

 the histological characters of ova, then the process may fairly enough be termed parthe- 

 nogenesis. 



