ON THE AGAMIC REPRODUCTION AND MORPHOLOGY OF APHIS OI 
day, it seems to me that the problem is reversed, and that the 
question before us is, why is sexual union necessary? Far from 
seeking for an explanation of the phenomena of gemmation in the 
transmitted influence of the spermatozoon, the philosopher acquainted 
with the existing state of science will seek, in the laws which govern 
gemmation, for an explanation of the spermatic influence. 
ParT II 
Read January 21st, 1858. 
§ 1. Embryogeny of the external organs of Aphis—$ 2. Embryogeny of AZyszs as exemplify- 
ing the Crustacea—§ 3. Embryogeny of Scorpio as exemplifying the Arachnida— 
§ 4. Generalizations regarding the Embryogeny of the Ar?zce/ata, and Morphological 
Laws based on them—§ 5. The Embryogeny of Articelata, Mollusca and Vertebrata 
compared. 
§ 1. Embryogeny of the external organs of Aphis 
In the previous part of this paper I sketched so much of the 
development of the embryo of Ap/is as was indispensable to the clear 
understanding of its reproductive processes; but it appears to me 
that the bearings of the embryogeny of this Insect upon morphology 
render it worthy of a more attentive and detailed consideration. 
It would be well worth while, indeed, to trace out the develop- 
ment of all the organs of this remarkable animal ; but as I shall have 
for some months no leisure for labours involving so great an expendi- 
ture of time, I will content myself for the present with a notice of 
some of the leading features presented by the development of the 
external organs. 
I have already stated that one of the earliest changes in the germ 
of the young of the viviparous Aphis is the differentiation of its 
cellular mass into a central portion, which takes on the appearance 
and functions of a yelk, and which I termed “the pseudovitellus,” and 
a peripheral coat or layer, the blastoderm. The blastoderm next 
becomes thickened posteriorly; and in this thickening a division 
takes place from without inwards, so that it is separated into a 
posterior flap and an anterior portion, which are only continuous 
dorsally. It is the flap which is the rudiment of the abdomen, while 
that portion of the blastoderm against which it is folded stands in 
the same relation to the thorax. In front of this is the rudiment of 
the head, constituting by far the largest portion of the blastoderm. 
