212 PROFESSOR HUXLEY ON THE ANATOMY 
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 T5T n< i of an i ncn > ana f° r the greater part of its extent it has the character of 
indifferent tissue. But the sudden enlargement to which I have referred is occupied by a 
body which has all the characters of an ovum, consisting of a structureless yelk - 6 -i^th of 
an inch in diameter, and of a clear germinal vesicle CraWth)* enclosed in which is a germinal 
spot (ijooth). It will appear by-and-by that this is, in fact, the solitary ovum (sur- 
rounded 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. 15 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 consti- 
tutes 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 ^ioth of an inch in diameter. The clear 
germinal vesicle measures -g-ooth 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. 
Eig. 16 represents a bud ^-^th °f an i ncn broad hy ^ioth of an inch high. The pro- 
cess 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 
increased in length to xi^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 distinguishable into a body or 
rudimentary ascidiozooid rrath 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 hol- 
low 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 pyri- 
fonn sac, somewhat similar in contour to the rudimentary ascidiozooid itself. The upper 
