CASTLE : EMBKYOLOGY OF CIONA INTESTINALIS. 271 



Seeliger ('85), Davidoff ('89), aud Samassa ('94) all state that the 

 first equatorial plane of cleavage in the ascidian egg separates the two 

 primary germ layers. According to my definition of the primary germ 

 layers in Ciona, this is not true, for several of the cells composing the 

 chorda-mesenchyme ring (included by me in tlie primary endoderm) are 

 derived from the four ventral cells, which according to their view are 

 exclusively ectodermal. The statement tliat the first equatorial plane of 

 cleavage separates the two primary germ layers is equally untenable, if 

 tested by the definition of primary germ layers accepted by the authors 

 mentioned ; for they include in the primary endoderm the entire tneso- 

 derm, which I have shown to be derived chiefly from the four venti'al 

 cells, which produce the definitive ectoderm. 



B. The CcBlom Theory. 



The brothers Hertwig ('81) divided the higher Metazoa into two 

 groups according as the body cavity arises by a pair of outpocketings 

 of the primary endoderm enclosing an enteroccel between visceral and 

 parietal mesoderm layers, or by a simple splitting or moving apart of 

 cells in a solid mass of mesoderm, which is then said to enclose a 

 schizoccel. The Chordates were unhesitatingly placed by them among 

 the Enteroccelians, and Amphioxus was cited as a typical example. The 

 Tunicates were thus classed as Enteroccelians, though no one had ever 

 observed in their ontogeny the formation of an enteroccel. Van Bene- 

 den et Julin ('86) supplied the lack by their studies on Clavelina ; but 

 considerable doubt has been thrown on the accuracy of their observa- 

 tions by the independent researches of Seeliger ('85) upon an undeter- 

 mined species of the same genus, and by those of DavidofF ('91) upon 

 the identical species studied by Van Beneden et Julin. Neither Seeliger 

 nor Davidoff could detect a trace of enteroco3l formation in the ontogeny 

 of Clavelina, and Davidoff was equally unsuccessful in finding an enteroccel 

 in Distaplia. My own observations on Ciona are entirely in agreement 

 with those of Seeliger and Davidoff on this point. Van Beneden et 

 Julin, notwithstanding their belief that an enteroccel is formed in Ascid- 

 ians, as well as in Amphioxus, rejected the classification of the brothers 

 Hertwig on other grounds. 



Lwoff ('94) has I'ecently shown that in Amphioxus the cavities enclosed 

 by outpocketings of the wall of the gastral cavity are evanescent struc- 

 tures, and have nothing to do with the subsequently formed body cavity, 

 which, as in all Vertebrates, arises by a wandering apart of mesoderm 



