ON THE AGAMIC REPRODUCTION AND MORPHOLOGY OF APHIS 67 
numerous close-set vesicular endoplasts, having a diameter of ;Jygth 
to sjoath of an inch. These usually contained many granules, some- 
times only one; but I cannot say I have been able to detect any 
definite nucleolus in them. 
The discoid blastoderm is thickest in its middle region, thinning 
off gradually on both sides, and internally is sharply defined from the 
substance of the vitellus In the centre it exhibits a more or less 
marked depression. As development goes on, this depression be- 
comes more and more marked, while the disk thickens and increases 
circumferentially. At the same time, the layer of yelk in immediate 
contact with the disk, and co-extensive with it, is found to have a 
somewhat different constitution from the rest. The globules are 
large, dark, and sharply defined, and acetic acid gives them a granular 
appearance, but developes no endoplast. 
The depression above alluded to now increases, so as ,to form a 
fissure which separates a small tongue-shaped process from the rest 
of the blastoderm, to which it nevertheless remains closely applied. 
This process is the rudiment of the abdomen, and in a front view it is 
rendered more distinct by several clear lines, which mark the com- 
mencement of the future caudal bristles. In front of the end of the 
abdominal process, two minute conical prominences, at first marked 
by similar, but fewer clear stria, gradually raise themselves up on 
each side from the surface of the blastoderm and elongate, their 
apices being directed backwards. They are the rudiments of the 
antennules and antenne. 
A delicate structureless membrane is now visible, covering these 
parts and the adjoining portions of the germinal membrane. It is 
produced into the terminal sete of the end of the abdomen and of 
the two pairs of appendages, and is the commencement of the first 
skin of the larva. 
The anterior part of the blastoderm is wider than the posterior, 
and is produced into two great lobes divided by a median fissure. 
These are the “ procephalic lobes,’ and have the same relation to the 
anterior division of the head as the corresponding parts in the embryo 
Aphis. 
In this state the embryo becomes a larva, for it bursts its vitelline 
envelope and lies naked in the pouch of the mother. The rudi- 
mentary abdomen is at the same time extended, so that the little 
creature is now about ;\;th of an inch in length, and is very like a 
pear in shape, the stalk being represented by the abdomen, which is 
terminated by a flattened, bifid, spinulose fin. 
1 See, for illustrative figures of the development of AZyszs, my ‘ Lectures’ above cited. 
F 2 
