466 DEVELOl'MENT AND EMBR YOLOQ T. 



a primary aud gradual manner. Many insects, and espec- 

 ially certain crustaceans, show ns 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, stud- 

 ded with polypi, and attached to a submarine rock, sliould 

 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 animal- 

 cules, which attach themselves to rocks and become devel- 

 oped 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, produc- 

 ing asexually other larv^, 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 larva3 of this fly having acquired 

 the power of asexual reproduction. As long as the case 

 remained nnique 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 cer- 

 tain extent, ^* unites that of the Cecidomyia with the 

 parthenogenesis of the Ooccidae;'' the term parthenogen- 

 esis 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 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 marvelous case of the Cecidomyia. 



It has already been stated that various parts in the same 

 individual, which are exactly alike during an early embry- 



