ON THE AGAMIC REPRODUCTION AND MORPHOLOGY OF APIIIS 37 
endoplast of not more than ;>}g,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 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 facts 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 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 nature, I have no doubt; 
and | therefore look upon it as an established fact, that the primary 
steps in the agamic development of Apfizs 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 
pseudovarium ; thirdly, the disappearance of the vesicle of the pseud- 
ovum, 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 any- 
thing 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— 
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 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 this be the case, however, the second hypothesis is excluded, and 
the third is improbable in itself, and is supported by no evidence. 
In the absence of such evidence, the marked contrast in size and 
appearance between the epithelial endoplasts of the penultimate 
