234 PROFESSOR HUXLEY ON THE ANATOMY 
constitute the ascidiozooids (Plate XXXI. figs. 11 and 12), each segment is about -r^th 
of an inch long by as much broad, and has a thickness of less than s^o^h of an inch. 
Like the blastoderm whence it proceeded, the segment appears to consist of nothing but 
a dense, opake mass of indifferent tissue. 
In a somewhat more advanced condition, the first signs of organization appear in the 
form of a clear median longitudinal streak visible in each segment when it is viewed from 
above. The streak is bounded by two more-opake lines, and on each side of the whole is 
a more opake mass. If the foetus be turned, so as to display a transverse section of one 
of the segments, the clear streak is seen to correspond with a central cavity answering to 
the alimentary tract of a bud, while the more opake lateral masses are plainly small sacs 
the lateral atria. The isthmus between any one segment and the next is clear in the 
middle, and has every appearance of a tube connecting the alimentary tracts of the two 
segments ; but if, as I have already said, the first isthmus enables the alimentary tract of 
the first ascidiozooid to communicate with the cavity of the cyathozooid, then the cavities 
of all the alimentary tracts of the ascidiozooids must be, indirectly, in communication with 
this cavity and, through it, with the exterior. In point of fact, I believe that the four 
primary ascidiozooids stand in the same relation to the cyathozooid, as four buds formed 
from the ascidiozooids in the way described above would do, if, in the process of gemma- 
tion as many remained connected together and with the parent ; for, as we have seen, 
all the branchial sacs of the buds communicate with that of the parent and, by the latter, 
wit h the exterior. And the mode of connexion of the different ascidiozooids is exactly the 
same in the two cases ; for, in somewhat more advanced foetuses (in which the ascidiozooids 
are about Tooth of an inch long and broad), it is obvious that the clear streak above men- 
tioned corresponds with the interval between the bands of the endostyle, and that the 
end of the alimentary tract of any one embryonic ascidiozooid which is continued into the 
isthmus corresponds with the endostylic cone of ordinary buds ; while that part of any 
embryonic ascidiozooid which receives an isthmus is the interval between the oesophageal 
aperture and the ganglion, just as this is the place into which the peduncle of a bud opens. 
In ascidiozooids of this size, the nature of what I have termed the lateral atria is 
demonstrated by the appearance of four or five stigmata in their inner wall, just as in 
buds at a corresponding stage. At the same time, that part of the indifferent tissue of 
the embryo which lies in the immediate vicinity of the pointed end of the alimentary 
tract (the future endostylic cone) becomes converted into a mass of clear reticulated 
tissue, the elseoblast {<b). This body is developed more largely laterally than in the 
middle line, so that it appears, at first, as if it were composed of two distinct portions ; 
but its two moieties are really continuous with one another on the hsemal side of the 
alimentary tract. The position of the future oral aperture is just indicated in the middle 
of the exposed surface of the ascidiozooids in this stage ; but I could not ascertain anything 
definite as to the condition of the intestine. Indeed, from the flattened form of the embry- 
onic ascidiozooids and their close apposition to the ovisac, it is exceedingly difficult to 
decipher all the details of their internal structure. 
Ascidiozooids of -f 6 th. of an inch in length exhibit a well-defined, though not open oral 
aperture, gioth of an inch in diameter. The branchial stigmata have increased m 
