220 PROFESSOR HUXLEY ON THE ANATOMY 



the ovisac have ample room within the sinus of the zooid in which they are lodged ; but 

 as they increase in size, the duct of the ovisac extending towards the neural side and for- 

 wards, and the duct of the testis extending toAvards the neural side and backwards, push 

 the atrial tunic before them, so that their openings are eventually situated on slight 

 papillary elevations. The principal portions of the two organs, on the other hand, con- 

 sisting of the sac of the ovisac and the ca^ca of the testis, as they enlarge, pass into cham- 

 bers in the test, which are formed for them by the recession of the outer tunic, and 

 whose cavities, consequently, communicate freely with the haemal blood-sinuses. 



With respect to that part of the generative blastema which remains in connexion with 

 the endostylic cone, one of its endoplasts or nuclei soon acquires a larger size, and 

 l)ecomes surrounded by a clear space, thus giving rise to a new germinal vesicle and 

 spot, round which will eventually be formed the solitary ovum and ovisac of a new bud, 

 developed from the zooid, Avhosc origin has just been traced, in exactly the same way as 

 itself has arisen. 



Thus, if we start with a single ascidiozooid, it may give rise, to all appearance, to an 

 indefinite succession of buds, by successive enlargements and detachments of the end 

 of the peduncle of the first ; and each bud thus developed carries within itself, in its 

 generative blastema and endostylic cone, provision for an indefinite succession of other 

 buds. It must be recollected, however, that while the tissue of the rudiments of the 

 alimentary and reproductive systems of each bud is directly descended, with compara- 

 tively little alteration, from the blastoderm of the embryo Pyrosoma, yet this tissue 

 cannot be said to be embryonic ; the tissue of the endostylic cone being considerably 

 cUff'erentiated, while the outer tunic of each bud is derived from the still more modified 

 outer tunic of the parent ascidiozooid. These facts, therefore, lend no countenance to the 

 doctrine, whose fallacy I have demonstrated in a previous memoir, that budding depends 

 on a retention of the primitive tissue of the germ in any part. 



§ -i. The Gamogenesis, or Sexual Bevelopment, o/Pyrosoma giganteum (Plate XXXI.). 

 It will conduce to intelligibility, if the somewhat complex history of this process is 

 divided into stages, characterized partly by the size of the ovisac, partly by its structural 

 characters. I shall describe, under each stage, a specimen or specimens, illustrating the 

 peculiar features of that stage, but it will be understood that insensible gradations are 

 observable between the diff'erent stages ; and, in order that the whole process of develop- 

 ment may be viewed continiiously, it will be advisable to consider, as the first stage, that 

 condition of the ovisac in which it is first recognizable as a completely distinct organ, a 

 condition which it attains, as I have ali*eady stated, in buds such as that figured in 

 PL XXX. fig. 23. 



First Stage. Ocisacs less than -g\-(yth of an i?ich in diameter and loitliout ducts. 

 Eig. 1, PI. XXXI., represents an ovisac measuring 4 loth of an inch in diameter. It is 

 ellipsoidal in form, and nowhere presents any prolongation whicli can be regarded as even 

 the rudiment of a duct. The wall of the ovisac is comparatively thick, and obscurely 



