AND MORPHOLOGY OF APHIS. 225 



§ 2. Embryogeny ©/"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 Artictilata, 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 E/athke'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 wUl therefore describe the development of Mysis, the Opossum-shrimp. 



The fertilized ova of this crustacean have a diameter of ^th to -^th of an inch, and 

 consist of a yelk enclosed within a colourless and tliin, but strong vitelline membrane. 



The yelk is composed of two elements — small and large yelk-masses, the former having 

 about -s^o^th to 4 u^o o^h of an inch average diameter, and being usually so closely 

 wedged together as to appear polygonal. The latter are large (xoVotb of an inch or more), 

 spherical, and imbedded in the mass formed by the smaller kind of yelk-granules. 



I was imable 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 t, or of acetic acid. When 

 the latter reagent has been employed, or even before, if the examination be very care- 

 folly 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 iinely 

 granular ; and imbedded within it are numerous close-set vesicular endoplasts, having a 

 diameter of rsVoth to 2 0^0 o^h 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 viteUus. 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 circumferentiaUy. 

 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 fissui-e 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 commencement 



* I have no doubt that the Myriapoda 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 uusatisfactorv 

 in the existing accounts of their development, and so many points in their anatomy requiring re-investigation, that I 

 prefer for the present to be silent about them. 



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. 



