Chap. XIV.] DEVELOPMENT AND EMBRYOLOGY. 387 



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 generation 

 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 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 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 larva?, 

 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 possible to account 

 for the larva? 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 flj*, 

 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 Coccida? ;" — 

 the term parthenogenesis implying that the mature females of the 

 Coccidoa are capable of producing fertile eggs without the con- 

 course of the male. Certain animals belonging to several classes 

 are now known to have the power of ordinaiy reproduction at an 

 unusually early age ; and we have only to accelerate partheno- 

 genetic 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 mar- 

 vellous case of the Cecidomyia. 



It has already been stated that various parts in the same indi- 

 vidual which are exactly alike during an early embryonic period, 

 become widely different and serve for widely different purposes in 

 the adult state. So again it has been shown that generally the 

 embryos of the most distinct species belonging to the same class 

 are closely similar, but become, when fully developed, widely dis- 

 similar. A better proof of this latter fact cannot be given than 

 xhe statement by Von Baer that " the embryos of mammalia, ol 



2 c 2 



