On the Larval Theory of the Origin of Tiatiae. 193 



u.\. 



iXII. — Larval Theory of the Origin of Tissue. 

 By A. Hyatt *. 



I HAVE endeavoured, in tlie essay of whicli this is an abstract, 

 to demonstrate a pliyletic connexion between Protozoa and 

 Metazoa, and also to show that the tissue- cells of the latter 

 are similar to asexual larva? and are related by their modes of 

 development to the Protozoa, just as larval forms among the 

 Metazoa themselves are related to the ancestral adults of the 

 different groups to which they belong. This is indicated by 

 the fact that the tissue-cells exhibit highly concentrated or 

 accelerated modes of development according to a universal law 

 of biogenesis, which has now been found in almost all groups 

 of animals. Thus in forms which stand at the extreme limits 

 of groups in point of specialization of structure, or have un- 

 usually protected young, or pathological forms with stimulated 

 development — in fact any forms in which stimulative causes 

 have acted upon the young, so as to bring about an earlier 

 development at the expense of the normal rate of growth — 

 there may be observed an abbreviation of the usual series of 

 structural characters, which appear in the young of normal 

 forms of the same group. The observations of many authors, 

 notably Cope, Hackel, Balfour, Weissmann, Packard, and 

 Wurtemburger, have conclusively proved that examples of 

 abbreviated or concentrated development are the results of a 

 constant tendency in all organisms to acquire characters in 

 adults or later stages of larvge, and then to inherit these at 

 earlier and earlier stages in successive descendants ; thus 

 finally crowding the younger stages until some ancient cha- 

 racters are skipped, sometimes leaving no record of the deri- 

 vation of the organism, and at others only a highly abbreviated 

 record in the earlier stages. 



No bushy colonies of zoons or cells are built up in the 

 Metazoa, representing the incompletely divided colonies of the 

 adults of Protozoa, except in cases of incomplete segmentation 

 of the ovum. These forms are skipped^ and the complex 

 colonies, which arise by fission, consist of zoons divided by 

 distinct walls. The cycle of transformations is not only 

 shortened by this omission, but the origin of the reproductive 

 bodies is carried back into the earlier stages in many forms^ 



* From the ' American Journal of Science ' for May 1886, pp. 332-347. 

 This article is an abstract of a paper with the same title published in Proc. 

 Bost. Soc. Nat. Hist. vol. xxiii. 1884, pp. 45-163, but has in addition the 

 suggestion that Volvo.r and Endorina are true iuteruiediate forms entitled 

 to be called Me.iOzoa or Blastrea. 



