§355. 



THE INSECTA. 



465 



mass, and is, theiefore, quite unlilse a proligeroug 

 disc. 



When these egg-like germs have attained the size 

 of one one-hundred-and-flftieth of an inch in di- 

 ameter, there begins to appear distinctly the sketch- 

 ing or marking out of the future embryo. This 

 sketching consists at first of delicately-marked re- 

 treatings of tlie cells here and there ; but these last 

 soon become more prominent from sulcations, and, 

 at last, the form of an articulated embryo is quite 

 prominent. 



During this time, the yellowish, vitellus-looking 

 mass has not changed its place, and although it is 

 somewhat increased in size, yet it appears other- 

 wise the same. When the development has pro- 

 ceeded a little further, and the embryo has as- 

 sumed a pretty definite form, the arches of the 

 segments, which have hitherto remained gapingly 

 open, appear to close together on the dorsal sur- 

 face, thereby enclosing the vitellus-looking mass 

 within the abdominal cavity. It is this same vitel- 

 loid mass thus enclosed, which furnishes the de- 

 velopment of the new germs (which in this case 

 would be those of the fourth colony, or D), and this 

 germ development here commences with the clos- 

 ing up of the abdominal cavity, and then the same 

 processes we have just described are repeated. 



The details of the development subsequent to 

 this lime, — the formation of the different systems 

 of organs, &c., are precisely like those of the de- 

 velopment of true oviparous Arthropoda in gene- 

 ral ; and although the ovoid germ has, at no time, 

 the structural peculiarities of a true ovum, — such 

 as a real vitellus, germinative vesicle and dot, yet 

 if we allow a little latitude in our comparison and 

 regard the vitellus-looking mass as the mucous, 

 and the germ-mass proper as the serous fold of 

 the germinatmg tissue, as in true ova ; if this com- 

 parison of parts can be admitted, then the analogy 

 of the secondary phases of development between 

 these forms, and true ova of the Arthropoda, can 

 be traced to a considerable extent. 



These secondary phases of development need not 

 here be detailed, for they correspond to those de- 

 scribed by Herold, Kblliker, of the true ovum in 

 other Insecta, and which, too, I have often traced in 

 various species of the Arthropoda in general. 



When the embryo is fully formed and ready to 

 burst from its capsule in which it has been de- 

 veloped, it is about one-sixteenth of an inch in 

 length, or more than eight times the size of the 

 germ, when the first traces of development in it 

 were seen. From this last-mentioned fact, it is 

 evident that, even admitting that these germ-masses 

 are true eggs, the conditions of development are 

 quite different from those of the eggs of the truly vivi- 

 parous animals, for, in these last, the egg is merely 

 hatched in the body instead of out of it, and, more- 

 over, it is formed exactly as though it was to be 

 deposited, and its vitellus contains all the nutritive 

 material required for the development of the em- 

 bryo until hatched. With the Aphididae, on the 

 other hand, the developing germ derives its nutri- 

 tive material from the fatty liquid in which it is 



bathed, and which fills the abdomen of the parent. 

 The conditions of development in this respect, are 

 here, therefore, more like those of the Mammalia and 

 the whole parent animal may be regarded in one 

 sense as an individualized uterus filled with germs, 

 — for the digestive canal with its appendages 

 seems to serve only as a kind of laboratory for the 

 conversion of the succulent liquids this animal ex- 

 tracts from the tree on which it lives, into this fatty 

 liquid which is the nutritive material of the germs. 



Omitting the curious and interesting details of 

 the further history of the economy of these Insecta, 

 as irrelevant to the point in discussion, we will now 

 turn to see what view we should take of these pro- 

 cesses, and what is their physiological interpreta- 

 tion. In the first place it is evident that the germs 

 which develop these viviparous Aphides are not 

 true eggs ; they have none of the structural char- 

 acteristics of these last, — such as a vitellus, a 

 geritiinative vesicle and dot ; on the other hand 

 they are at first simple collections, in oval masses, 

 of nucleated cells. Then again, they receive no 

 special fecundating power from the male, which is 

 the necessary preliminary condition of all true 

 eggs ; and furthermore the appearance of the new 

 individual is not preceded by the phenomena of 

 segmentation, as is also the case with all true eggs. 

 Therefore, their primitive formation, their develop- 

 ment and the preparatory changes they undergo 

 for the evolution of the new individual, are all dif- 

 ferent from those of real ova. 



Another point of equal importance is these vivi- 

 parous individuals of the Aphides have no proper 

 ovaries and oviducts. Distinct organs of this kind 

 I have never been able to make out. The germs, 

 as we have before seen, are situated in moniliform 

 rows, like the successive joints of confervoid plants, 

 and are not enclosed in a specirl tube. These rows 

 of germs commence, each, from a single germ-mass 

 which sprouts from the inner surface of tlie animal, 

 and increases in length and. the number of its com- 

 ponent parts by the successive formation of new 

 germs by the constriction process as already de- 

 scribed. Moreover, these rows of germs which, at 

 one period, closely resemble in general form, the 

 ovaries of some true Insecta, are not continuous 

 with any uterine or other female organ, and there- 

 fore do not at all communicate with the external 

 world, on the other hand, they are simply att;ichfd 

 to the inner surface of the animal, and their com- 

 ponent germs are detached into the abdominal 

 cavity as fast as they are developed, and thence 

 escape outwards through a Porus genitalis. 



With these data, the question arises, what is the 

 proper interpretation to be put upon these repro- 

 ductive phenomena we have just described ? My 

 answer would be that the whole constitutes only a 

 rather anomalous form of gemraiparity ; as already 

 shown, the viviparous Aphididae are sexless ; they 

 are not females, for they have no female organs, 

 they are simply sremrniparous, and the budding is 

 internal, instead of external as with the Polypi and 

 Acalephae ; moreover this budding takes on some of 

 the morphological peculiarities of oviparity but those 



