EMBRYOLOGY 397 



minute, single, and very simple eye-spot. In this last and 

 complete state, cirripedes may be considered as either 

 more highly or more lowly organised than they were in 

 the larval condition. But in some genera the larva 

 become developed either into hermaphrodites having 

 the ordinary structure, or into what I have called com- 

 plement^ males : and in the latter, the development 

 has assuredly been retrograde ; for the male is a mere 

 sack, which lives for a short time, and is destitute of 

 mouth, stomach, or other organ of importance, except- 

 ing for reproduction. 



We are so much accustomed to see differences in 

 structure between the embryo and the adult, and like- 

 wise a close similarity in the embryos of widely different 

 animals within the same class, that we might be led 

 to look at these facts as necessarily contingent in some 

 manner on growth. But there is no obvious reason why, 

 for instance, the wing of a bat, or the fin of a porpoise, 

 should not have been sketched out with all the parts 

 in proper proportion, as soon as any structure became 

 visible in the embryo. And in some whole groups of 

 animals and in certain members of other groups, the 

 embryo does not at any period differ widely from the 

 adult : thus Owen has remarked in regard to cuttle-fish, 

 ' there is no metamorphosis ; the cephalopodic character 

 is manifested long before the parts of the embryo are 

 completed ; ' and again in spiders, ' there is nothing 

 worthy to be called a metamorphosis.' The larvae of 

 insects, whether adapted to the most diverse and active 

 habits, or quite inactive, being fed by their parents or 

 placed in the midst of proper nutriment, yet nearly all 

 pass through a similar worm-like stage of development; 

 but in some few cases, as in that of Aphis, if we look to 

 the admirable drawings by Professor Huxley of the 

 development of this insect, we see no trace of the 

 vermiform stage. 



How, then, can we explain these several facts in 

 embryology, — namely the very general, but not uni- 

 versal difference in structure between the embryo and 

 the adult; — of parts in the same individual embryo, 



