W. K. BEOOKS ON THE GENUS SALPA. 303 



As the eleoblast of the solitary salpa is median and unpaired, the 

 separation of its rudiment into halves in the aggregated salpa is, like the 

 separation of the pharynx into paired pharyngeal pouches, a secondary 

 adaptation to the conditions which are imposed by the peculiar method 

 of budding. The morphological middle line of the body is occupied by 

 the blood tubes of the stolon, and the rudiment of the stoloblast has thus 

 been pushed to the right and to the left until it has become completely 

 divided, during the ontogenetic history, into halves. 



The point has theoretical interest. The stoloblast of the aggregated 

 salpa is unquestionably homologous with the eleoblast of the solitary 

 salpa, and is, therefore, a degenerated tail, as I have shown on page 38, 

 but the eleoblast of the solitary salpa is median, and the cause of its 

 separation into halves in the aggregated form is clear. 



Davidoff has shown (16), p. 585, that since the middle line of the body 

 of the distaplia embryo is occupied by the blastopore when the notochord 

 and nervous system are first differentiated, these arise as paired organs. 

 The history of the eleoblast of salpa shows that we cannot regard this as 

 proof that the notochord and nervous system are phylogenetically paired, 

 for the same reasoning would force us to regard the pharynx of salpa as, 

 in origin, a paired organ. 



SECTION 13. The Ovary and Testis. 



The origin and the homology of the ovary of salpa have been fully 

 discussed in Chapters IX and X, and I have nothing more to add. 



In my first paper on salpa (The Development of Salpa, Bull. Mus. 

 Comp. Zool, 1876, No. 14) I advanced the opinion that the testis of the 

 chain-salpa is homologous with the eleoblast of the solitary salpa, a view 

 which is, of course, totally untenable at present. Salensky was almost 

 equally unfortunate in the view which he advanced a year later (3), for 

 he derives the genital rod of the stolon from the eleoblast of the solitary 

 salpa, and holds that the testis of the chain-salpa is not derived from the 

 genital rod, but that it arises, in place, from " mesoderm " cells. 



Seeliger, nearly ten years later (11), p. 74, showed that the testis 

 actually arises from the follicular epithelium of the genital rod, and 

 my own observations show that this view is correct, although I have on 

 p. 230 given my reasons for rejecting Seeliger' s theoretical deductions 

 from its history. 



