102 HISTORY OF CRUSTACEA. CHAP. X. 



rious structure of the vitelline membrane, the varying 

 composition of the vitellus, the different number and 

 formation of the germinal spots, &c. Numerous exam- 

 ples, which might easily be augmented, of such pro- 

 found differences, are furnished by Leydig's ' Lehrbuch 

 der Histologie.' In the Crustacea the ovarian eggs 

 actually sometimes furnish excellent characters for the 

 discrimination of species of the same genus ; thus, for 

 example, in one Porcellana of this country they are 

 blackish-green, in a second deep blood-red, and in a 

 third dark yellow ; and within the limits of the same 

 order they present considerable differences in size, 

 which, as Yan Beneden and Glaus have already pointed 

 out, stands in intimate connexion with the subsequent 

 mode of development. 



" The organs of the l}ody are formed in the sequence of 

 their organic importance; the most essential always 

 appear first." This proposition might be characterised 

 a priori as undemonstrable, since it is impossible either 

 in general, or for any particular animal, to establish a 

 sequence of importance amongst equally indispensable 

 parts. Which is the more important, the lung or the 

 heart ? the liver or the kidney ? the artery or the 

 vein ? Instead of giving the preference, with Agassiz, 

 to the organs of animal life, we might with equal 

 justice give it to those of vegetative life, as the latter 

 are conceivable without the former, but not the former 

 without the latter. We might urge that, according to 

 this proposition, provisional organs as the first pro- 

 duced must exceed the later-formed permanent organs 

 in importance. 



