428 DEVELOPMENT AND EMBRYOLOGY. 



crustaceans, show us what wonderful changes of structure 

 can be effected during development. Such changes, how- 

 ever, reach their acme in the so-called alternate generations 

 of some of the lower animals. It is, for instance, an aston- 

 ishing fact that a delicate branching coralline, studded with 

 polypi, and attached to a submarine rock, should produce, 

 first by budding and then by transverse division, a host of 

 huge floating jelly-fishes ; and that these should produce 

 eggs, from which are hatched swimming animalcules, which 

 attach themselves to rocks and become developed into branch- 

 ing 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 strength- 

 ened 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, propagating their kind in the ordinary 

 manner by eggs. 



It may be worth notice, that when Wagner's remarkable 

 discovery was first announced, I was asked how was it pos- 

 sible 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, repro- 

 duces 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 Coccidae ; " the term parthenogenesis implying that the 

 mature females of the Coccidae are capable of producing 

 fertile eggs without 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 repro- 

 duction 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. 



It has already been stated that various parts in the same 

 individual, which are exactly alike during an early embry- 

 onic period, become widely different and serve for widely 

 different purposes in the adult state. So again it has been 

 fhown that general^ tlie embryos, pf the most; distinct sp$- 



