338 ON THE ANATOMY AND DEVELOPMENT OF PYROSOMA 
free in the cavity of the blood-sinus, at a distance of ;1,th of an inch 
from the ovisac of the ascidiozooid, which is 51,th of an inch in 
diameter, and so far advanced that there can be no question as to its. 
real nature. The total length of this mass (which, for reasons which 
will shortly appear more fully, I have termed the generative blastema) 
is z},nd of an inch; and for the greater part of its extent it has the 
character of indifferent tissue. But the sudden enlargement to which 
T have referred is occupied by a body which has all the characters of 
an ovum, consisting of a structureless yelk g15th of an inch in 
diameter, and of a clear germinal vesicle (;s5 5th), enclosed in which 
is a germinal spot (zJ;,th). It will appear by-and-by that this is, in 
fact, the solitary ovum (surrounded by its rudimentary ovisac) which 
will come to maturity in the bud to be formed at this spot ; and it is 
not a little remarkable that the first recognizable part of the new 
organism should be the foundation of that structure which will 
eventually develope into a creature distinct from it. 
In fig. 55 a more advanced condition of a bud is depicted; the 
backward continuation of the endostylic cone is broader, more 
distinctly hollow, and is so bent up as to form a more acute angle, 
both with the line of direction of the endostyle and with the plane of 
the generative blastema. In consequence of this change, and of the 
general enlargement of the parts, they can no longer be contained 
within the blood-sinus, whose outer wall is now elevated into a conical 
cap which fits over the conjoined ends of the process of the endostyle 
and the generative blastema. That part of the external tunic which 
constitutes this cap is thickened, and exhibits the texture of indifferent 
tissue. The ovum in the generative blastema is now very distinct, 
and the tissue around it is so disposed as to mark out the walls of an 
ovisac which measures ;3,th of an inch in diameter. The clear 
germinal vesicle measures ;goth of an inch ; and its spot has the same 
diameter as before. Behind the ovisac, which occupies the greater 
part of the cavity of the diverticulum constituting the bud, a distinct 
constriction marks off the rest of the generative blastema, which lies 
closely connected with the external tunic of the parent, and altogether 
excluded from the cavity of the “cap” of the nascent bud. It is now 
no longer taper, but cylindrical and rounded at the end; and near its. 
anterior extremity a new germinal spot, surrounded by a small clear 
vesicle, is visible. 
Fig. 16 represents a bud 535th of an inch broad by 735th of an 
inch high. The process of the endostylic cone is very distinctly 
hollow and somewhat thin-walled, while its axis is nearly parallel 
with that of the bud. In fig. 17, the bud, now subcylindrical, has 
