DEVELOPMENT OF THE OVUM. 139 



He states his views thus : "I look upon it as an 

 established fact, that the primary steps in the agamic 

 development of Aphis are : First, the enlargement 

 of the periplast around one of the pseudovarian vesicles 

 and its detachment as a separate body, which from its 

 resemblance to an ovum I will call a pseudovum. 

 Secondly, the contemporaneous formation of a distinct 

 chamber — the penultimate chamber of the pseudovum. 

 Thirdly, the disappearance of the vesicle of the 

 pseudovum, and the conversion of the latter into a 

 germ-mass composed of cells embedded in intercellular 

 substance.* 



2.— DEVELOPMENT OF THE OVUM. 



The examination of the ova of Aphides after laying is 

 attended with some difficulty, on account of their dense 

 and highly coloured shells. Usually when first excluded 

 these tough membranes are pale and yellowish, but they 

 speedily become brown or black; a change which com- 

 monly takes place after impregnation. No light passes 

 through them in this condition, but a treatment with 

 olive oil would seem to render them less opaque, and 

 if precautions be taken the maceration is not neces- 

 sarily attended by the death of the egg.f However, 

 some of the changes within are much more readily 

 studied before than after extrusion from the ovaries. 



I have made many dissections of Aphides for 

 my own satisfaction, and some of the drawings I ven- 

 ture to place before the reader in Plates H and I. 

 Notwithstanding their simplicity they may not only 

 give confirmatory evidence, but here and there per- 

 chance suggest something of interest to the practised 

 morphological eye. 



The letterpress in connection with these plates will 



* T. H. Huxley, "Morphology of Aphis," 'Trans. Linn. Soc.,' vol. 

 xxiii, p. 201. 

 1 f Balbiani, ' Mem. d. Gen. des Aph.' 



