66 ON THE AGAMIC REPRODUCTION AND MORPHOLOGY OF APHIS 
Neuroptera. And, considering the universality of the law that the 
embryogenic processes of members of the same class have a similar 
fundamental character, I do not doubt that the development of all 
insects is, in its main features, a process similar to that described 
in Aphis. 
§ 2. Embryogeny of Mysis as exemplifying the Crustacea. 
But more than this, if we extend our researches into the em- 
bryogeny of the other two principal! classes of the Artecudata, 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 knowledge, and I will therefore 
describe the development of JZys7s, the Opossum-shrimp. 
The fertilized ova of this crustacean have a diameter of #;th to 
isth 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 zji5pth to zgsgth of an inch average 
diameter, and being usually so closely wedged together as to appear 
polygonal. The latter are large (;,455th 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 developes 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 trans- 
mitted light. The contrast is greatly heightened by the addition of 
alcohol,? or of acetic acid. When the latter reagent has been em- 
ployed, or even before, if the examination be very carefully 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 embedded within it are 
1 T have no doubt that the AZyrzapoda will be found to exemplify the same morphological 
laws, with the exception of that relating to the total number of somites in the body, as their 
congeners ; but I find so much that is unsatisfactory in the existing accounts of their develop- 
ment, and so many points in their anatomy requiring re-investigation, that I prefer for the 
present to be silent about them. 
* 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. 
