1892.] 



Development of Ciona intestinalis, fyc. 



519 



with, certainty, although, from the appearances presented, and also 

 from the analogy of Clavelina, it is probable that it arises in the same 

 •way in both cases. 



The formation of the heart presents interesting differences in the 

 two forms. In Clavelina, as already shown by van Beneden and 

 Julin, the septum which, at first, divides the pericardium into two 

 halves breaks down, and the heart forms as an involution of the 

 dorsal wall of the pericardium. In Ciona the septum does not break 

 down, and the heart forms by a splitting apart of the two layers 

 which compose the septum. 



Conclusions. 



What has been said above is enough to show that the development 

 of Ciona presents much more primitive features than that of Clavelina. 

 It now remains to compare the conditions in Ciona with those that 

 obtain in Amphioxus, and to seek to establish the true homologies 

 between the various parts. 



In Amphioxus, what I propose to call the proboscis-cavity is lined 

 by a flat epithelium, and so is the rest of the body-cavity. In Ciona, 

 the proboscis-cavity contains loose mesoderm cells in place of an 

 epithelium, and so does the rest of the body-cavity. The distinction 

 between mesoderm and mesenchym is no longer generally recognised 

 as fundamental. 



The presence of the prae-oral pit as a pair to the proboscis-cavity 

 of Amphioxus seems, at first sight, to present a difficulty in the way 

 of the comparison which I am making; but it is nob so serious as 

 might be supposed, and, for the rest, I need only refer here to what 

 occurs in different species of Balanoglossus. 



In instituting any comparison between the Ascidians and Amphi- 

 oxus, the endostyle should be taken as the starting point, and the 

 fact should be remembered that its primary axis is perpendicular to 

 its definitive axis in both cases. 



Making allowance for the secondary change of position which the 

 mouth has undergone in the larva of Amphioxus, in correlation with 

 the forward extension of the notochord, we find, therefore, that the 

 relative position of the various organs from before backwards is pre- 

 cisely the same in Ciona and in Amphioxus, namely, (1) proboscis- 

 cavity, (2) endostyle, (3) mouth, (4) first pair of gill-slits. 



It should be remarked that the mesoderm which lies in the pro- 

 boscis-cavity of Ciona has a bilateral origin, corresponding more 

 or less closely to the pair of anterior intestinal diverticula of 

 Amphioxus. 



It is most important to establish the homology of the cavities of 

 Ciona and Amphioxus on a sound basis. The endostyle admittedly 

 occupies the same position primarily in both animals. 



