196 W. K. BROOKS. 



In all other respects Davidoff' s observations are a complete 

 confirmation 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 seg- 

 mentation to be discovered, and that there is not the least indication 

 of any cavity comparable to a myocoel. The embryonic history 

 of the mesoderm of distaplia cannot be referred back in any way 

 whatever to anything comparable to Hertwig's conception 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 the mesoderm is two-layered, so that a parietal and 

 a visceral layer may be distinguished, there is no homology 

 between these layers and the bounding walls of the coelom of the 

 enterocoelomata. ' ' 



After reviewing all the literature on the subject, he gives as the 

 general result of his studies the statement (p. 622) that " the body 

 cavity of the ascidians lies between the two primary germ layers 

 and must be regarded as a blastocoel, which would be identical with 

 the segmentation cavity if this were not temporarily obliterated 

 during gastrulation by the contact of the ectoderm cells and endo- 

 derm cells." 



While the salpa-embryo is very complicated and unfavorable 

 for studying this question, my own observations, which have 

 already been described, seem to show that the body cavity of 

 salpa is, like that of clavelina and distaplia, a primary one, 

 fundamentally identical with the segmentation cavity, and that 

 the mesoderm arises as free mesenchyma cells derived from the 

 endodermal blastomeres. 



The body cavity of the salpa-embryo is identical with the space 

 between the somatic and visceral layers of follicle cells, and while 

 there is a stage in which these two layers are in contact, the 

 follicular cavity is undoubtedly the same as the cavity shown at 

 15 in Plate XI, Fig. 3, of my memoir on salpa, and this is the 

 same as the space which is shown in Plate X, Fig. 3, between the 

 segmenting egg and the follicle. 



