Hyatt] 160 [March 5, 



had a similar homoplastic simplicity of structure. The develop- 

 ment in Ascidia of the notochordal cells and muscle cells from 

 the walls of the archenteron invite the suggestion, that no true 

 diverticula exist in this type. That the lateral muscles might 

 have arisen as entirely disconnected and 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. 2, f. 10-13). In this Hydrozoon portions of the 

 archenteric walls grow out and become directly converted into 

 muscles, but no ccelom is formed. The notochord may have primi- 

 tively 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 1 . 



The homoplastic origin of the notochord, when explained in this 

 way agrees with the subsequent origin of segmentation in the 

 vertebrae as suggested by Cope. These facts and agreements in 

 theory render it highly probable, that the whole phenomena of seg- 

 mentation as shown in the distribution of the muscles themselves, 

 the appendages and internal organs, including even the primitive 

 somites, may have arisen independently in the Vertebrata in re- 

 sponse to the simple mechanical requirements of motion in elon- 

 gated bodies. Herbert Spencer in a treatise much neglected by 

 naturalists (Prin. Biol., Am. Ed., 1871, vol. n, p. 199), has clearly 

 shown that the origin of the notochord and of segmentation of 

 the vertebrae 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 Haeckel in so far they dis- 

 tinctly point to the gastrula and planula as the last stages which 

 have a general genetic meaning, and show that these indicate a 

 stock form for the whole animal kingdom. 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 ap- 

 pearance invariably after the planula or gastrula, and never under 

 any conditions break this natural succession gives strong support 



1 We desire in this connection to correct the erroneous impressions likely to arise 

 from the note regarding antero-posterior symmetry on page 107. We by no means 

 meant to be committed to an assertion that the antero-posterior correspondence of the 

 segments could be considered as in any sense the same as the lateral correspondences. 



