and Embryogeny of the Aphides, 67 



culata, containing only the elements of the head and of the in- 

 ferior wall of the body. The sexual masses are therefore, in 

 point of fact, completely naked and outside the embryo. But 

 already we can see that the embryonic cells arrange themselves 

 in parallel series directed towards the extremity of the abdo- 

 men, to form excretory ducts, destined to place them in relation 

 with that region. 



Development continuing under these conditions, the embryo 

 enlarges, and with it the whole sexual apparatus; the parts al- 

 ready existing become completed and perfected, the mouth and 

 anus are formed, and the digestive tube becomes visible at its 

 extremities. It is at this point that a phenomenon occurs 

 equally simple in its mechanism and important in its results 

 for the further progress of the development of the Articulata, 

 namely the reversal of the mode of rolling of the embryo. This 

 reversal, which is not always effected by means of exactly the 

 same process in all these animals, takes place in the Aphides by 

 an actual backward summerset executed by the embryo in the 

 interior of its cell. In consequence of this change of position 

 the head, which at first was in relation to the posterior part of 

 the cell, arrives at the anterior part, while the ventral surface, at 

 first turned inwards, now looks outwards, and is placed imme- 

 diately under the envelope of the egg. By the same act the 

 abdomen is transferred to the dorsal side, and rises, like a sort 

 of tail to the embryo, as far as the posterior part of the head, 

 leaving between it and the cephalothoracic rudiment a space, 

 the greater part of which is occupied by the mass of the gene- 

 rative organs. In this new position all that is necessary is that 

 the embryo should complete itself behind by the formation of a 

 dorsal wall, in order that these organs may be enclosed in the 

 cavity of the body quite naturally and without any new change 

 of position. 



The closure of the hinder part of the body is effected by the 

 simple growth of the ventral arches towards the dorsal region, 

 and their fusion in the median line of the latter. 



If we endeavour at this period of development to under- 

 stand the arrangement of the hermaphrodite apparatus of the 

 viviparous Aphides, we find the primitive common mass of ova- 

 rian cells divided into two groups, symmetrically placed in the 

 posterior part of the body, and each of these groups formed of 

 a small number of cellular masses, each of which possesses a 

 proper envelope. In these we easily recognize the terminal 

 chambers of the ovarian sheaths, with their contents, consisting 

 of small transparent cells. The male organ is likewise divided 

 into two parts, arranged, in the form of two cords of variable 

 form, on each side of the digestive tube, within the ovaries. 



