[ 221 ] XV. On the Agamic Reproduction and Morphology of Aphis. — Part II. By Thomas H. Huxley, F.R.S., Professor of Natural History, Government School of Mines. Communicated by G. Busk, F.B.S., F.L.S. Read January 21st, 1858. § 1 . Embryogeny of the external organs of Aphis. § 2. Embryogeny oi Myais as exemplifying the Crustacea. § 3. Embryogeny of Scorpio as exemplifying the Arachnida. § 4. Generalizations regarding the Embryogeny of the Articulata, and Morpho-logical Laws based on them. § 5. The Embryogeny oi Articulata, Mollusca and Fertebrata compared. § 1. Embryogeny of the external organs q/" Aphis. In the previous part of this paper I sketched so much of the development of the embiyo 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 diiferentiation 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 tliickening 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. 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 the interval is occupied by the pseudovitellus, which here comes into immediate contact with the pseudovitelline membrane. In an embiyo xe o^h of an inch ia length (PI. XXXVII. fig. 5), this interval has increased so much, that the cephalic blastoderm does not extend on to the dorsal region at aU, but lies abnost flat under the pseudovitellus, in the anterior half of the ventral region. In embryos jiotli of an inch in length (PL XXXVII. fig. 6.), I have foimd the 2 G 2
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