200 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



from a series of pairs of gut-pouches, and that the simplicity of appendi- 

 cularia cannot be primitive, inasmuch as the ancestors of the tunicates 

 once possessed these complicated structures. 



The first step to take in discussing this objection is to learn whether 

 there are any traces of gut-pouches in the tunicates. 



Seeliger (p. 9) has given us a very minute and detailed account of the 

 history of the mesoderm in the clavelina embryo, and has shown that it 

 arises from two rows of endoderm cells which give origin, in the tail, to 

 the caudal muscles and, in the body, to free mesoderm cells which 

 multiply with great rapidity and wander everywhere through the body 

 cavity, which is bounded on one side by the endodermal wall of the gut, 

 and on the other by the ectoderm. 



He says emphatically (p. 128) that the mesoderm arises as two 

 totally unsegmented rows of cells, each forming a single layer ; that the 

 body cavity is not an enterocosl, but a primary body cavity; and that the 

 ontogeny of the tunicate mesoderm gives no evidence of derivation from 

 paired pouches comparable to the coelomic pouches of amphioxus. 



It is a rare thing for students of tunicate morphology to agree, but 

 in this case the phenomena are simple, and Davidoff (p. 16) completely 

 confirms Seeliger's observations, so far as they bear upon the question, 

 by his own studies of clavelina and distaplia. 



His account of the origin of the mesoderm differs from Seeliger's in 

 only one minor point, which has no bearing upon the question under 

 consideration. Like Seeliger, he derives the mesoderm from two rows 

 of endoderm cells, but he says that these cells remain as endoderm cells 

 after they have given rise to the mesoderm, while Seeliger states that 

 they become converted into the mesoderm. 



In all other respects Davidoff s observations are a complete confir- 

 mation of Seeliger's, for he says (600) that in distaplia the mesoderm 

 of the caudal region persists as a solid rudiment and becomes the 

 muscular layer of the tail, while elsewhere it breaks up into wandering 

 mesenchyma cells. "It is to be particularly emphasized that in no part 

 of the mesoderm is any trace of segmentation to be discovered, and that 

 there is not the least indication of any cavity comparable to a myoccel. 

 The embryonic history of the mesoderm of distaplia cannot be referred 

 back in any way whatever to anything comparable to Hertwig's concep- 

 tion of the enterocoelomata." 



Of clavelina he says (607) : " There is not even a transitory division 

 of the mesoderm into a somatopleur and a splanchnopleur. Even where 



