234 PROFESSOR HUXLEY ON THE ANATOMY 



constitute the ascidiozooids (Plate XXXI. figs. 11 and 12), each segment is about yi^tb 

 of an inch long by as much broad, and has a thickness of less than g-^oth 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, 

 with 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 xoo^li 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 w^all, 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 heemal 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 --j^qUi of an inch in length exhibit a well-defined, though not open oral 

 aperture, gioth of an inch in diameter. The l^ranchial stigmata have increased in 



