34- 



OVUM. 



process among animals higher in the scale, but 

 in them of an exceptional character, has long 

 been known to occur in the various species of 

 the common plant-louse, or Aphis, first dis- 

 covered by Reaumur * and Bonnet f, and 

 confirmed and more fully illustrated by a 

 variety of accurate entomologists in more 

 recent times. In this animal, successive gener- 

 ations, amounting each to a considerable num- 

 ber, and in the Aphis lanigera J averaging 

 about a hundred, are produced for seven, nine, 

 or eleven times, according to the species, 

 from parents of no sex, or rather which seem 

 to possess the structure of females imper- 

 fectly developed. The course of the gene- 

 rative process is the following : perfect male 

 and female winged insects are observed only 

 towards the autumn season : these fly about 

 in great quantities, the impregnated females 

 deposit their eggs covered with a protecting 

 case of mucus in the axils and other recesses 

 of plants, where they remain during the winter. 

 In spring there are developed from these ova a 

 brood of larvae, or imperfect female Aphides, 

 which soon produce, by an act of viviparous 

 generation, and without any concurrence of the 

 male sex, a progeny of a similar kind, and this 

 is repeated, in successive generations, for nine 

 or ten times in the common species, or for 

 ten or twelve weeks during the summer, at 

 the end of which time the last brood brings 

 forth male and perfect female individuals, 

 both of which die after having provided by 

 the production of fecundated ova for the con- 

 tinued generation during the next season. 



Upon the discovery of this very remarkable 

 mode of reproduction, various theoretical con- 

 jectures were made in regard to its nature ; 

 but no satisfactory explanation presented 

 itself, till the knowledge of the general 

 nature of the process of non-sexual larvation 

 came to be brought under a general principle 

 or law. It is now obvious that the production 

 of the successive generations of Aphis-larvae 

 may be regarded as an instance of the multi- 

 plication of individuals from the product of 

 a single ovum, previous to the development of 

 the true sexual organs and the exercise of 

 the sexual functions. But this example of non- 

 sexual larvation deserves attention, not only 

 from its occurrence among animals placed so 

 high in the zoological scale of organisation as 

 insects, but also from the degree of perfection 

 of the larvae themselves, and from the circum- 

 stance that the new broods are formed, not 



recent observations, the parent or stock individuals 

 of both Syllis prolifera and Nais proboscidea arrive 

 ultimately at sexual perfection alter having given 

 off a number of sexual individuals by the caudal 

 gemmation. See Leuckart on Metamorphosis, Non- 

 sexual Reproduction and Alternate Generations, in 

 Zeitsch. fur Wissensch. Zool. 1851, in which he 

 refers to Frey and Leuckart, in Beitrage zur Kennt- 

 niss Wirbellos. Thiere, p. 96 ; and to Schultze, in 

 Arehiv. fur Naturgesch, 1849, p. 287, for observa- 

 tions proving this fact. 



* Histoire cles Insectes, Tom. iii. Paris, 1738. 



f Traitt; d'Insectologie ou Observations sur les 

 Pucerons, 8vo, 1745. 



J See Owen's Lect. on the Invertebrate Animals, 

 1848, p. 235. 



as in the other examples of this process to 

 which the attention of the reader has already 

 been directed, by a division of the whole 

 body, or by gemmation from its external or 

 internal substance, but from germs arising 

 within a determinate organ, corresponding m 

 its general form and anatomical relation, 

 though not entirely similar, to the generative 

 oraan of the perfect female.* 



The genital orcans of the viviparous or larval 

 Aphides differ from those of the perfect or 

 oviparous females, principally in the want of 

 the receptaculum seminis, and the organ which 

 secretes the mucous investment for the ova, 

 and there is also some difference in the form 

 of the ovarium, or germiparous part of the 

 organs.f 



The gradual development of the larva brood 

 within the oviduct of the viviparous parent 

 has been traced carefully by several observers. 

 J. Victor Cams has attempted to show that 

 this process is to be distinguished from the 

 usual process of development from an ovum by 

 the absence of cell multiplication, and by the 

 formation of the embryo of the larva from a 

 merely granular germ : but more recently 

 Ley dig J has shown that though there may be 

 differences in the structure and mode of de- 

 velopment of the ovum and of the viviparous 

 germ, the latter arises as truly as the former 

 from cellular elements. The uppermost com- 

 partment of the oviduct contains, according to 

 Leydig, from eight to twelve distinct nu- 

 cleated cells, together with a quantity of 

 finely granular substance. One of these cells 

 appears to descend into the second compart- 

 ment, in which an outer layer of cells exists, 

 with finely granular substance internally : in the 

 third compartment the cells of the outer layer 

 have become still smaller and more numerous, 

 and have formed, in fact, a covering to the germ, 

 similar to that which proceeds from cleavage 

 in the ovum. The rudiments of the internal 

 organs now begin to be distinguishable, and 

 the various external organs are successively 

 developed out of the cellular mass. The cel- 

 lular structure of the ovum of insects is at all 

 times difficult to be traced in the earlier 

 stages of development ; and it seems to be 

 established by these observations that the 

 origin of the embryonic structures from cells 

 is at least as obvious in the viviparous as in 

 the oviparous germ (see^g. 30. R.). 



General Remarks on Alternate Generations. 

 Having now shortly described the principal 

 varieties of the reproductive process which 

 may be brought under Steenstrup's law ol 

 " Alternate Generation," it may be proper to 

 review very briefly their general nature. 



* It is conjectured by Yon Siebold (Vergleich 

 Anat. der Wirbellos. Thiere, p. G34), that the same 

 mode of production may occur also in some kinds 

 of Cynips and Psyche. 



f 'See the interesting observations of Owen, ir 

 Parthenogenesis, p. 38., and elsewhere ; V. Siebold 

 Vergleich. Auat. and Froriep's Neue Notizen, vol 

 xii. p. 308 ; and J. Victor Carus, Ziir niiherr 

 Kenntniss des Gcnerations-Wechsels, Leipzig, 184.9 



J V. Siebold and Kb'lliker's Zeitsch. fiir Wissen. 

 sehaftliche Zoologie, vol. ii. p. 53. 



