and Embryogeny of the Aphides. 63 



and their relations to the rest of the organism. But, in the first 

 place, it is necessary to give an idea of the structure presented, 

 according to my observations, by the organ in which the embryo 

 originates — that is to say, the female sexual apparatus or ovary of 

 -the viviparous Aphides. 



This apparatus consists, as in most other insects, of a variable 

 number of tubes or sheaths, each of which is dilated at its 

 anterior extremity into a terminal cell or chamber containing a 

 group of small cells. One of these occupies the centre of the 

 group and is entirely surrounded by the others. This central 

 cell is the most important of all ; for it represents the generative 

 element or the mother cell of all the ovules in each sheath which 

 are destined to become converted into embryos. These ovules 

 originate in the form of true buds, which, separating successively 

 from the central cell, appear at the bottom of the terminal 

 chamber before passing into the upper part of the sheath. 

 The peripheral cells, attached to the former by hollow pedicles, 

 are its nutritive cells, its sole function being to emit incessantly 

 new ovular buds. 



At the moment when the ovule penetrates into the ovarian 

 sheath, it clearly presents a germinal vesicle and spot. A delicate 

 filament still attaches it for a time to the mother cell ; but this 

 union is soon broken, and the ovule remains completely isolated 

 in its chamber. It is generally at this moment that the modi- 

 fications which are to lead to the formation of the embryo 

 commence in the ovum. The germinal spot first disappears, and 

 is soon followed by the vesicle which contained it. During this 

 period some nuclei, at first rare, have made their appearance on 

 the surface of the vitellus, and condensed around them the 

 transparent homogeneous substance of which it is composed. 

 By this means are formed the first blastodermic cells. No 

 membrane as yet surrounds them. The rather wide intervals 

 separating them at first are quickly filled up by the appearance 

 of new nuclei and cells. The ovule is thus finally clothed over 

 the whole of its surface by a continuous layer of cells arranged 

 in a single series and pressed against each other. At this period 

 they all present a very recognizable proper envelope. 



While the blastoderm has thus been completing its formation, 

 the ovum has increased in size and passed from a spherical form 

 to that of an elongated oval ; at the same time it has descended 

 a little in the interior of the ovarian sheath. The central vitel- 

 line mass enclosed in the cavity of the blastoderm has lost its 

 homogeneous aspect and become penetrated by fine colourless 

 granulations. Soon afterwards an orifice is formed at the pos- 

 terior pole of the blastoderm*, in consequence of the separation at 



* I give the name of posterior pole of the blastoderm or ovum to the 



