536 Professor Huxley, on [May 21, 



The species Aphis, therefore, is fully manifested not in any one 

 being or animated form, but by a cycle of such, consisting of, — Ist 

 tlie egg; 2nd, An indefinite succession of viviparous Aphides; 

 3rd, Males and females eventually produced by these, and giving 

 rise to the egg again. 



If, armed with the microscope and scalpel, we examine into the 

 minute nature of these processes (without which inquiry all specu- 

 lation upon their nature is vain), we find that the viviparous Aphis 

 contains an organ similar to the ovarium of the oviparous female, in 

 some respects, but differing from it, as Von Siebold was the first 

 to show, in the absence of what are termed the colleterial glands 

 and the spermatheca — organs of essential importance to the oviparous 

 form. 



In the terminal chambers of this " Pseud-ovarium," ovum-like 

 bodies, thence called " pseud-ova," are found. These bodies pass 

 one by one into the pseudovarian tubes, and there gradually 

 become developed into young, living Aphides. As Morren has 

 well said, therefore, the young Aphides are produced by " the indi- 

 vidualization of a previously organized tissue," 



The only organic operation with which this mode of develop- 

 ment can be compared, is the process of budding or gemmation, as 

 it takes place in the vegetable kingdom, in the lower forms of 

 animal life, and in the process of formation of the limbs and other 

 organs of the higher animals. And the parallel is complete if such 

 a plant as the bulbiferous lily or the Marchantia, or such an animal 

 as the Hydra, is made the term of comparison. 



Thus agamogenesis in Aj)his, is a kind of internal budding 

 or gemmation. If we inquire how this process differs from multi- 

 plication by true ova or " Gamogenesis," we find that the young 

 ovum in the ovarium is also, to all intents and purposes, a bud, 

 indistinguishable from the germ in the pseudovarium of the agamo- 

 genetic Aphis. Histologically there is no difference between the 

 two ; but there is an immense qualitative or physiological difference, 

 which cannot be detected by the eye, but becomes at once obvious 

 in the behaviour of the two germs after a certain period of their 

 growth. Dating from this period, the pseudovum spontaneously 

 passes into the form of an embryo, becoming larger and larger as it 

 does so ; but the ovum simply enlarges, accumulates nutritive 

 matter, acquires its outer investments, and then falls into a state of 

 apparent rest, from which it will never emerge, unless the influence 

 of the spermatozoon have been brought to bear upon it. 



That the vast physiological difference between the ovum and 

 the pseudovum should reveal itself in the young state by no external 

 sign, is no more wonderful than that primarily the tissue of the 

 brain should be undistinguishable from that of the heart. 



The phenomena which have been described, were long supposed 

 to be isolated, but numerous cases of a like kind, some even more 

 remarkable, are now known. 



