332 A. Hyatt — Larval Theory of the Origin of Tissue. 



Abt. XXXI. — Larval Theory of the Origin of Tissue , by A. 



Hyatt.* 



I have endeavored in the essay of which this is an abstract 

 to demonstrate a phyletic connection between Protozoa and 

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

 similar to asexual larvae 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 de- 

 velopment 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, Hseckel, Balfour, Weissmann, Packard and Wurtem- 

 burger have conclusively proved that examples of abbreviated 

 or concentrated development are the results of a constant tend- 

 ency in all organisms to acquire characters in adults or later 

 stages of larva? and then to inherit these at earlier and earlier 

 stages in successive descendants; thus finally crowding the 

 younger stages until some ancient characters are skipped, some- 

 times leaving no record of the derivation 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 col- 

 onies, 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 reproduc- 

 ductive bodies is carried back into the earlier stages in many 

 forms, and the rapidity of the processes of complete fission due 

 to concentration produces masses of tissue and membranes 



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

 Soc. Nat. Hist., vol. xxiii, 1884, p. 45-163, but has in additiou the suggestion 

 that Volvox and Budorina are true intermediate forms entitled to be called 

 Mesozoa, or Blastrea. 



