ON THE ANATOMY AND DEVELOPMENT OF PYROSOMA 367 
long by as much broad, and has a thickness of less than ;4,th of an 
inch. Like the blastoderm whence it proceeded, the segment appears 
to consist of nothing but a dense, opaque mass of indifferent tissue. 
In a somewhat more advanced condition, the first signs of organi- 
zation 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-opaque lines, and on each side of the whole is a more 
opaque 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 acentral cavity answering to the alimentary tract of a bud, while 
the more opaque 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 gemmation 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, with the 
exterior. And the mode of connexion of the different ascidiozooids 
is exactly the same in the two cases; for, in somewhat more ad- 
vanced fcetuses (in which the ascidiozooids are about 35th of an inch 
long and broad), it is obvious that the clear streak above mentioned 
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 em- 
bryonic ascidiozooid which receives an isthmus is the interval 
between the cesophageal 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 ali- 
mentary tract (the future endostylic cone) becomes converted into 
a mass of clear reticulated tissue, the eleoblast (@). This body is 
