AND MORPHOLOGY OF APHIS. 225 
; $ 2. Embryogeny of Mysis as exemplifying the Crustacea. 
But more than this, if we extend our researches into the embryogeny of the other two 
principal* classes of the Articulata, the Arachnida and Crustacea, we shall see that it 
presents a most remarkable agreement with that of the Insect. 
To illustrate this important truth, I might cite Rathke’s account of the development of 
Astacus as a type of crustacean embryogeny; but I prefer to speak from my own know- 
ledge, and I will therefore describe the development of Mysis, the Opossum-shrimp. 
The fertilized ova of this crustacean have a diameter of #sth to yyth of an inch, and 
consist of a yelk enclosed within a colourless and thin, but strong vitelline membrane. 
The yelk is composed of two elements—small and large yelk-masses, the former having 
about 3560th to zoooth of an inch average diameter, and being usually so closely 
wedged together as to appear polygonal. The latter are large (tooth of an inch or more), 
spherical, and imbedded in the mass formed by the smaller kind of yelk-granules. 
I was unable to detect any trace of endoplasts or cells in these ova, Acetic acid deve- 
lopes neither granules nor endoplasts in the yelk-masses. Upon the yelk thus constituted, 
the blastoderm makes its appearance as a rounded patch, which reflects the light more 
than the yelk, and therefore appears white by reflected, and dark by transmitted light. 
The contrast is greatly heightened by the addition of alcohol}, or of acetic acid. When 
the latter reagent has been employed, or even before, if the examination be very care- 
fully conducted, the structure of the blastoderm is seen to be widely different from that 
of the rest of the yelk. No yelk-granules are visible in it, but it appears to be very finely 
granular; and imbedded within it are numerous close-set vesicular endoplasts, having a 
diameter of +3455th to z555th 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 
becomes 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 
å 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 commencement 
* I have no doubt that the Myriapoda will be found to exemplify the same morphological laws, with the exeeption 
of that relating to the total number of somites in the body, as their congeners ; but I find so much that rige 
in the existing accounts of their development, and so many points in their anatomy requiring re-investigation, 
Prefer for the present to be silent about them. i 
T Rathke, in his numerous embryological researches, appears to have constantly availed himself of this property of 
alcohol in order to render the blastoderm more distinct. 
