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 fora 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 
* T give the name of posterior pole of the blastoderm or ovum to the 
