AND MORPHOLOGY OF APHIS. 201 



Finally, the vesicle itself ceases to be visible (fig. •!•), and the penultimate chamber 

 contains only its epithelium and a mass of apparently structui'eless substance ; — I say 

 apparently structureless, because the addition of water made the mass more clear, and at 

 the same time rendered an irregular aroolation and scattered granules visible in its sub- 

 stance. Whether the areolas are the outlines of delicate vesicles, and the granules theu' 

 endoplasts, or not, are points which I could not satisfactorily determine ; at any rate, I 

 could never observe anything like the regular structure observable in the contents of this 

 chamber when a little larger. 



Fig. 5 represents such a chamber, 4Tyth of an inch in length. The cndojilasts of 

 the wall are seen lying in or upon it, and occupying its interior is a distinct oval mass of 

 substance agreeing in appearance with the periplast of the pseudovum, but distinguished 

 from it by containing a great number of clear spheroidal cavities not more than s-sVot^' 

 of an inch in diameter, each of which contains a central endoplast of not more than 

 iftooo th of an inch. These cavities are closely packed, but not flattened against one 

 another. The walls of the cavities react differently on the addition of acetic acid to the 

 rest of the periplast, becoming darker and more sharply defined. In fact, each ca\rity 

 is Avhat is commonly termed a nucleated cell, while the intervening periplast is the so- 

 called intercellular substance. 



I have here stated merely the histological facts which may be observed by any one 

 who wiU take the trouble to examine with sufiicient care the ultimate and penultimate 

 pseudovarial chambers of a few viviparous Aphides. Of the existence of these states, 

 and that the order in which I have detailed them fairly represents the order in which 

 they succeed one another in natvu'e, I have no doubt ; and I therefore 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 owim, I will call a " pseudovum;" 

 secondly, the contemporaneous formation of a distinct chamber- — the penultimate cham- 

 ber of the pseudovarium ; thirdly, the disappearance of the vesicle of the pseudo^iun, and 

 the conversion of the latter into a germ-mass composed of cells imbedded in intercellular 

 substance and containing minute endoplasts. 



I should be sorry, however, to express an opinion as to the exact nature of the process 

 by which these changes are effected, with anything like tlie same degree of confidence. 

 Three hypotheses present themselves : — 



1st. The pseudoval endoplast divides and subdivides, so as to give rise to the endoplasts 

 of the germ ; or — 



2nd. The pseudoval endoplast is resolved, and the endoplasts of the germ are developed 

 autogenously in its periplast ; or — 



3rd. The pseudoval endoplast disappears, and the endoplasts of the germ are supplied 

 from the epithelium of the walls of the pseudovarial chamber. 



Of these three hypotheses, I strongly incUne towards the fii'st, as most in accordance 

 with what we know of histological development in general. The whole progress of 

 modern research, in fact, goes to show that cells and endoplasts hardly, if ever, ai-ise 

 autogenously, but are the result of the siibdi\-ision of pre-existing cells and endoplasts. If 



