CASTLE: EMBRYOLOGY OF CIONA INTESTINALIS. 211 
Seeliger (85), Davidoff ('59), and 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 the primary endoderm) are 
derived from the four ventral cells, which according to their view are 
exclusively ectodermal. The statement that the first equatorial plane of 
eleavage 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 meso- 
derm, which I have shown to be derived chiefly from the four ventral 
cells, which produce the definitive ectoderm. 
B. The Colom 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 enterocel 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 
schizocel. The Chordates were unhesitatingly placed by them among 
the Enterocolians, and Amphioxus was cited as a typical example. The 
Tunicates were thus classed as Enteroeodians, though no one had ever 
observed in their ontogeny the formation of an enterocol. 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 enteroco formation in the ontogeny 
of Clavelina, and Davidoff was equally unsuccessful in finding an enterocool 
in Distaplia. My own observations on Ciona are entirely in agreement 
with those of Seeliger and Davidoff on this point. Wan Beneden et 
Julin, notwithstanding their belief that an enterocal is formed in Ascid- 
ians, as well as in Amphioxus, reje the classification of the brothers 
rtwig on other erounds. 
Lwoff (94) has recently 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 
