ON THE ANATOMY AND DEVELOPMENT OF PYROSOMA = 339 
increased in length to z1;nd of an inch; and the front view of a 
similar bud, given in fig. 18, shows that the hollow process of the 
endostylic cone is slightly constricted in the middle, and that the 
interval between its walls and the external tunic is occupied by a 
granular mass. 
In fig. 19, a marked advance is discernible. The bud is distin- 
guishable into a body or rudimentary ascidiozooid ;4sth of an inch 
long, and a much shorter stalk or peduncle. The ascidiozooid is 
broad at its attached end, more or less tapering at its opposite 
extremity. Its external tunic is distinct, but proportionably thinner 
than before, and is continued into the outer wall of the peduncle and 
thence into the external tunic of the parent. The hollow process of 
the endostylic cone is about as broad as before, in the peduncle ; but 
after traversing this, nearer its anterior than its posterior side, it 
suddenly dilates into a pyriform sac, somewhat similar in contour to. 
the rudimentary ascidiozooid itself. The upper taper end of this sac 
seems to be attached to the inner surface of the apex of the outer 
tunic of the ascidiozooid. Anteriorly and posteriorly, its walls appear 
thick, the enlargement being much more marked posteriorly. The 
side of the sac turned towards the eye, between these thickenings, 
exhibits five faint rings, with comparatively clear centres. In order 
to avoid circumlocution, I may so far anticipate the results yielded by 
the investigation of later stages of the buds, as to state the nature of 
the parts which have now been described. The rings are the indica- 
tions of the commencement of as many branchial stigmata; the 
anterior apparent thickening is the result of the formation of the 
rudiment of the endostyle; the posterior apparent thickening is 
produced by the rudiment of all that part of the alimentary tract 
which lies behind the branchial sac, into which almost the whole of 
the dilated end of the prolongation of the endostylic cone is converted. 
A comparatively clear space surrounds the apex of the branchial sac, 
below which the inner surface of the external tunic presents a band- 
like aggregation of indifferent tissue, the rudiment of a body which 
corresponds with what Krohn has called the elzoblast in the Safe ; 
and finally, projecting from the posterior wall of the external tunic, 
and apparently connected with the elzoblast, is an elongated mass, 
the anterior portion of the generative blastema, which has now become 
completely separated from the posterior part. The anterior end of 
the latter, in fact, extends only into the peduncle, while its posterior 
moiety lies, attached to the outer tunic of the parent, in the great 
hzemal blood-sinus. The generative blastema may therefore be now 
distinguished into three parts—parental, peduncular, and gemmular 
Z2 
