140 BRITISH APHIDES. 



supply sufficient information to the general reader to 

 carry him through the series. 



Ocular proof is easy to be obtained microscopically, 

 that the ovum and the pseudovum arise from organs 

 essentially similar. What has been before said of 

 the pseudovarium may, in almost the same terms, be 

 also said of the ovarium. 



The ovarian chamber has a translucent terminal 

 ca3cum, within which eight or more nucleated movable 

 masses are disseminated. These eventually pass down 

 into the constriction which occurs below, and probably 

 through or by means of the yolk-cord discovered by 

 Huxley, and more recently noted by Brandt. 



In passing the spermatheca (which does not occur 

 in the viviparous female) the ova meet with the issuing 

 spermatozoa ; or, as segmentation may be seen in the 

 eggs before they descend into the ovaries, it is possible 

 that the sperm- cells travel into the chamber above, 

 just before the egg makes its descent and issue. 



The body of the ovum is filled by a mass of active 

 living protoplasm, which exerts its influence on tho 

 food-yolk about it. The yolk-cells often congregate 

 more densely at the poles of the ovum, whilst tho 

 germinal vesicle, which is situated at the other end, is 

 freer from the same. The germinal spot usually occu- 

 pies the centre of the egg ; but, as the last matures, it 

 travels towards the side, where important changes set 

 in which are attendant upon fertilisation. 



The division of the blastoderm into membranes so 

 as to form a kind of loop has been already alluded to. 

 If the edges of this loop or protostome do not close, 

 only one opening is formed in the ectoderm ; but if 

 otherwise, two openings arise, and these form the 

 apertures of the alimentary canal. 



The localisation of function is carried on by cells 

 segregating or separating for definite ends. Somo 

 cells organise themselves for reproduction, others for 

 nutrition, and others for the production of scales, 

 wool, skin, and the like. 



