CHAP. XIV.] DEVELOPMENT AND EMBRYOLOGY. 363 



Development and Embryology. 



This is one of the most important subjects in the whole round 

 of natural history. The metamorphoses of insects, with which 

 every one is familiar, are generally effected abruptly by a few 

 stages; but the transformations are in reality numerous and 

 gradual, though concealed. A certain ephemerous insect (Chloeon) 

 during its development, moults, as shown by Sir J. Lubbock, above 

 twenty times, and each time undergoes a certain amount of change ; 

 and in this case we see the act of metamorphosis performed in a 

 primary and gradual manner. Many insects, and especially certain 

 crustaceans, show us what wonderful changes of structure can be 

 effected during development. Such changes, however, reach their 

 acme in the so-called alternate generations of some of the lower 

 animals. It is, for instance, an astonishing fact that a delicate 

 branching coralline, studded with polypi and attached to a sub- 

 marine rock, should produce, first by budding and then by trans- 

 verse division, a host of huge floating jelly-fishes ; and that these 

 should produce eggs, from which are hatched swimming animal- 

 cules, which attach themselves to rocks and become developed into 

 branching corallines ; and so on in an endless cycle. The belief 

 in the essential identity of the process of alternate generation and 

 of ordinary metamorphosis has been greatly strengthened by 

 Wagner's discovery of the larva or maggot of a fly, namely the 

 Cecidomyia, producing asexually other larvae, and these others, 

 which finally are developed into mature males and females, propa- 

 gating their kind in the ordinary manner by eggs. 



It may be worth notice that when Wagner's remarkable dis- 

 covery was first announced, I was asked how was it possible to 

 account for the larvae of this fly having acquired the power of 

 asexual reproduction. As long as the case remained unique no 

 answer could be given. But already Grimm has shown that another 

 fly, a Chironomus, reproduces itself in nearly the same manner, 

 and he believes that this occurs frequently in the Order. It is the 

 pupa, and not the larva, of the Chironomus which has this power ; 

 and Grimm further shows that this case, to a certain extent, 

 " unites that of the Cecidomyia with the parthenogenesis of the 

 Coccidse ; " the term parthenogenesis implying that the mature 

 females of the Coccidse are capable of producing fertile eggs with- 

 out the concourse of the male. Certain animals belonging to 

 several classes are now known to have the power of ordinary 

 reproduction at an unusually early age; and we have only to 

 accelerate parthenogenetic reproduction by gradual steps to an 

 earlier and earlier age, Chironomus showing us an almost exactly 

 intermediate stage, viz., that of the pupa and we can perhaps 

 account for the marvellous case of the Cecidomyia. 



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