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



more primitive structural elements than the coelomata is shown 

 by Kowalevsky's work on Cassiopea already quoted (Soc. 

 Friends of Nat. Hist., etc., Moscow, pi. II, f. 10-18). In this 

 Hydrozoon, portions of the archenteric walls grow out and be- 

 come directly converted into muscles, but no ccelom is formed. 



The notochord may have primitively originated as a tube, 

 but connections with the hypophysis seems to be a necessary 

 condition of this theory, and though this is highly probable, 

 it is not proven. The homoplastic origin of the notochord, 

 when explained in this way, agrees with the subsequent origin 

 of segmentation in the vertebras as suggested by Cope. These 

 facts and agreements in theory render it highly probable, that 

 the whole phenomena of segmentation as shown in the distri- 

 bution of the muscles themselves, the appendages, and internal 

 organs including even the primitive somites may have arisen 

 independently in the Yertebrata in response to the simple me- 

 chanical requirements of motion in elongated bodies. Herbert 

 Spencer in a treatise much neglected by naturalists (Prin. Biol., 

 Am. Ed., 1871, vol. ii, p. 199), has clearly shown that the 

 origin of the notochord and of segmentation of the vertebras 

 and muscles may be attributed to muscular strains, and our 

 speculations though entirely independent cannot lay claim to 

 any original merit. 



Our results are similar to those of Hseckel so far as they dis- 

 tinctly point to the gastrula and planula as the earliest stages 

 which have a general genetic meaning for the Metazoa and 

 show that these indicate a stock form for the whole of the Meta- 

 zoa. The clear distinctions between the type-larval stages in 

 different branches of the animal kingdom and the fact, that the 

 type larval stages make their appearance invariably after the 

 planula or gastrula, and never under any conditions break this 

 natural succession, give strong support to this opinion. 



It is possibly premature to say that no one type can be 

 claimed to have descended from any other, but the Porifera, 

 Hydrozoa, Actinozoa and Yertebrata appear to us entirely in- 

 dependent of each other. It is also very suggestive that two 

 so closely allied groups as the Actinozoa and Hydrozoa, can 

 be considered as homoplastic types and that many examples 

 have been brought forward by the author and Professor Cope* 

 among Cephalopoda and Yertebrata, where smaller and more 

 closely allied groups, orders, families and genera show the same 

 phenomena and are plainly homoplastic with reference to the 

 origin of many important characteristics of structure. These 

 results sustain the opinion that homogenous characteristics are 



* Cope, who first pointed out these relations in the same sense as Lank ester 

 used the terms "homologous" for homoplastic and "heterologous" for homo- 

 genous. 



