202 JOHNS HOPKINS UNIVEESITY MORPHOLOGICAL MONOGRAPHS. 



and a splanchnopleur, but Davidoff has shown by careful serial sections 

 that this statement is probably based upon erroneous observations. 



Salensky holds (17, 460) that the mesoderm of the blastoderm of 

 pyrosoma consists of two symmetrically placed ccelomic pouches, and 

 that pyrosoma is, therefore, to be placed among the true enterocoelomata. 

 The space between the vertebrate blastoderm and the yolk is undoubtedly 

 homologous with the enteron, but it is by no means certain that 

 this is the case in pyrosoma, where the food-yolk is an independent 

 acquisition ; nor do Salensky's figures show, as clearly as we might wish, 

 that the two coslomic vesicles open into this space, and even if this is the 

 case, we must remember that the pyrosoma-embryo is very aberrant, 

 and that the structure of its body cavity may be a secondary adaptation 

 to the presence of the yolk. Taken alone it certainly is not enough to 

 prove, without corroboration from other sources, that the body cavity of 

 the tunicate is an enterocoel. 



The ontogeny and homology of the tunicate mesoderm have been 

 recently discussed at very great length by Seeliger (11, pp. 85-104 and 

 pp. 126-131), by Davidoff (16, pp. 592-628), and by Salensky (17, pp. 456-462 

 and pp. 468-470 and 36^6), and as those who wish can find in these 

 papers an extended presentation of the complicated and perplexing 

 theory (?) of the mesoderm, I have attempted to treat it very briefly. 



The literature shows that there is no direct evidence whatever of the 

 existence, at any time in the history of the tunicates, of a metameric 

 series of coelomic pouches, and the supposed necessity for believing that 

 such a series existed in the primitive chordata is only another aspect of 

 the dogma that the metamerism of the vertebrates must have been 

 inherited from a primitive metameric ancestor. 



If, as I believe, the metamerism of vertebrates is secondary, the 

 metamerism of the mesoderm and body cavity may have resulted from 

 the duplication of a single pair of coelomic pouches similar to those of 

 echinoderm larvae, and it is quite conceivable that these may have been 

 acquired by the ancestors of the vertebrates after the divergence of the 

 tunicates. 



If, however, future research should show that there is a pair of gut- 

 pouches in the embryo of appendicularia, or should prove in some other 

 way that the structures which Salensky describes are true enterocosls 

 inherited from an ancestral tunicate, such a discovery, which is certainly 

 among the possibilities, would be no evidence that the primitive tunicate 

 was the degenerated descendant of an ancestor with metameric gut- 

 pouches. 



