170 LEG RS TITS, [VoL. XII. 
walls taking part in its formation are concerned. The over- 
lying test is not yet perforated, however. The great growth 
of the epithelium lining the newly formed siphon, resulting in 
its remarkable thickness, and the wide, circular double folds, 
$:., Sf, and sjf.1, 1s the fact to swhich I refer. Study) of 
various stages in this growth shows that all the thickened part 
of the epithelium is derived from the ectodermal invagination, 
and that the formation of the folds is due to growth of this 
ectodermal epithelium. The secondary fold, s.f.1, represents 
the evagination from the inner layer, it remaining passive, 
while the other portion grows rapidly, and produces, as the 
figure shows, a well-marked valve. As development goes on 
the whole siphon expands, and this fold or valve becomes 
narrower and narrower, and finally in the adult it is wholly 
obliterated, at least in the normal uncontracted condition 
of the animal. The formation of this fold is probably to 
permit of the protrusion of the siphon above the general level 
of the colony in its fully developed state. As shown by 
this figure, at this stage, z.e. previous to the perforation of the 
test, there is no such protrusion. This explanation hardly 
accounts, however, for the great height of the entire lining 
epithelium during these stages of development. 
A very different explanation suggests itself, though one too 
imaginative to be deserving of more than a mere mention. 
One might conjecture that as incident to the abbreviated 
development of the Ascidian blastozooid as compared with the 
embryonic development, the earliest, paired stage in the origin 
of the atrial opening, so well known in the larval history of 
many Ascidians, had been entirely lost in the development of 
the bud, and that the peculiar conditions above described rep- 
resent the stage in the formation of that opening immediately 
after the fusion of the two primitive paired embryonic orifices. 
Van Beneden et Julin (84), pp. 625-627, to whom we are 
indebted for the facts in the embryonal development above 
referred to, quite insist on the importance of distinguish- 
ing the cloaca proper from the peribranchial cavities. They 
point out that in Phallusia scabroides the cloaca is wholly 
lined by an ingrowth of ectoderm from the dorsal side 


