AND MORPHOLOGY OF APHIS. 199 



gical difference between the primary germs of the viviparous Aphis and true ova, — Carus 

 and Burnett reiterating their opinions even since the publication of Leydig's views. 

 Finally, Mr. Lubbock, in his late valuable memoir on Daphnia, (PhU. Trans. 1857) has 

 expressed his inability to find any germinal vesicle in the germs of the viviparous Aphis, 

 and, so far, may be ranked among Leydig's opponents. 



I have recently resumed some investigations commenced two or three years ago on 

 this interesting subject. My object was originally purely morphological, — the Aphis sug- 

 gesting itseK as a very convenient subject for working out the general development of 

 Insecta ; but I have found myself unable to refrain from wandering out of my dii-ect 

 course, and attempting to further the solution of the great problem of Agamogenesis, or 

 asexual I'eproduction. 



My observations are in the main in accordance with those of Leydig. On many 

 minor points, however, we are at variance ; and besides this, there are matters of great 

 interest, upon which Leydig does not touch, but on which I hope to 1)e able to thi-ow some 

 light. For, besides yielding an answer to the question as to the existence or absence of 

 any histological distinction between a bud and an ovum, the investigation of the ^-ivi- 

 parous and oviparous Aphides affords decisive evidence as to the soundness of certain 

 explanations of the phenomena of Agamogenesis in general ; and finally, the study of 

 the general development of Aphis furnishes data of great importance in Articulate Mor- 

 phology. 



I propose in the present memoir to follow out these lines of inquiry. I will in the first 

 place describe the minute structure of the essential reproductive organs or " Pseudovaria" 

 of the viviparous or agamic female ; and the development of its germs or pseudova (as I 

 propose to term them) will be considered. Secondly, the reproductive organs of the ovipa- 

 rous female and the development of the ova Tvdll be described. Next, I shall speak of the 

 manner in which the proliferous apparatus or pseudovarium of the viviparous female is 

 developed within the germ ; and I shall compare together the agamic and sexual repro- 

 ductive processes. I shall then endeavour by means of these facts to refute a hypothesis 

 wliich has been offered in explanation of Agamogenesis ; and finally, I propose to consider 

 the Morphology of the Articulata so far as it is elucidated by Development. 



The species of Aphis, the reproductive organs of whose viviparous form I am about to 

 describe, appeared this autumn upon a plant of the Ivy-leafed Geranium which hangs 

 in my study, and for the last two months has been regularly giving rise to broods, some- 

 times winged and sometimes apterous, without any appearance of males or females. 

 With respect to the external characters of the reproductive organs, I have nothing of 

 importance to add to Siebold's or Morren's description. 



§ 2. The Development of the Pseudovum. 



The terminal chamber of any of the caeca of the pseudovarium is a rounded or oval body 

 (Pl.XXXVI. fig. 1, A), united by a delicate ligament («), proceeding from its free end, Avith 

 the ligaments which pass from the other caeca of the same side, to form the common pseud- 

 ovarian ligament. The wall of the cliamber is a delicate transparent membrane (i), in which, 

 here and there, rounded endoplasts (or nuclei) are imbedded ; while ethyl's lie 6ti its inii^r 



