ON THE AGAMIC REPRODUCTION AND MORPHOLOGY OF APHIS 59 
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 
third 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 devoured. 
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 manifesta- 
tions, may fairly be attempted, and, when successfully carried out, 
will prove the first step towards an explanation. 
§ 7. Classification of the Phenomena of Agamogenests 
It does not seem to be very difficult to effect 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 individual), 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 Descontinuous Development. 
In continuous development, the size may increase, the form and 
texture remaining unchanged—constituting simple grow/¢h; or, the 
size remaining unchanged, the form and texture may alter—con- 
stituting simple metamorphosis; or the two processes may be com- 
bined, as in all those changes which we term gemation, without 
separation from the parent. 
Discontinuous development differs from continuous only in this, 
that the products of the growth and metamorphosis of the embryo 
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 without the influence of an act of conjugation 
with another zooid, I term the process agamogenests. The producing 
zooid may be devoid of sexual organs, as in the Sa/pe, many f{ydro- 
goa, many Trematoda—in fact, in the great majority of cases of 
agamogenesis. 
I term the first producing zooid of the individual the protosoozd ; 
"the produced zooids, dewterosootds. In some cases the deuterozooids 
acquire sexual organs, and give rise to ova and spermatozoa ; but in 
others they produce new zooids: thus broods of ¢rztozoords, &c., will 
