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XV. On the Agamic Reproduction and Morphology of Aphis.—Part II. By Tuomas 
H. Huxuzv, F.R.S., Professor of Natural History, Government School of Mines. 
Communicated by G. Busk, F.R.S., F.L.S. 
Read January 21st, 1858. 
$ 1. Embryogeny of the external organs of Aphis. 
$ 2. Embryogeny of Mysis as exemplifying the Crustacea. 
$ 3. Embryogeny of Scorpio as exemplifying the Arachnida. 
$ 4. Generalizations regarding the Embryogeny of the Artieulata, and Morpho- 
: logical Laws based on them. 
$ 5. The Embryogeny of Articulata, Mollusca and Vertebrata compared. 
$ 1. Embryogeny of the external organs of Aphis. 
IN the previous part of this paper T sketched so much of the development of the embryo 
of Aphis 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 development of all the organs of 
this remarkable animal; but as I shall have for some months no leisure for labours invol- 
ving so great an expenditure 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. : pe 
Dorsally and posteriorly, the rudiment of the head is originally continuous with the 
thoracico-abdominal thickening; but a separation early takes place at this part, and em 
interval is occupied by the pseudovitellus, which here comes into immediate contact wit 
the pseudovitelline membrane. | à 
In an embryo 745th of an inch in length (Pl. XXXVII. fig. 5), this interval has serre 
so much, that the cephalic blastoderm does not extend on to the dorsal region at all, bu 
lies almost flat under the pseudovitellus, in the anterior half of the ventral region. 
In embryos „15th of an inch in length (Pl. XXXVII. fig. 6), I zei Zi the 
