AND MORPHOLOGY OF APHIS. 201 
Finally, the vesicle itself ceases to be visible (fig. 4), and the penultimate chamber 
contains only its epithelium and a mass of apparently structureless substance ;—I say 
apparently structureless, because the addition of water made the mass more clear, and at 
the same time rendered an irregular areolation and scattered granules visible in its sub- 
stance. Whether the areolæ are the outlines of delicate vesicles, and the granules their 
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, zrrth of an inch in length. The endoplasts of 
the wall are seen lying in or upon it, and oceupying 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 elear spheroidal cavities not more than s3loth 
of an inch in diameter, each of which contains a central endoplast of not more than 
Tooooth 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 cavity 
is what is commonly termed a nucleated cell, while the intervening periplast is the so- 
called intercellular substance. 
I have here stated merely the histological faets which may be observed by any one 
who will take the trouble to examine with sufficient care the ultimate and penultimate 
pseudovarial chambers of a few viviparous.4phides. 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 nature, 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 
ås å separate body, which, from its resemblance to an ovum, 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 pseudovum, 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 the same degree of confidence. 
Three hypotheses present themselves :— € 
Ist. 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— : 
9rd. 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 incline towards the first, 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, arise 
autogenously, but are the result of the subdivision of pre-existing cells and endoplasts. If 
