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 towards 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 caeca 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 
becomes 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, whose 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 
differentiated, 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. 
§ 4. The Gamogeuesis, or Sexual Development, of 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 different stages ; and, in order that the whole process of develop- 
ment may be viewed continuously, 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 already stated, in buds such as that figured in 
PL XXX. fig. 23. 
First Stage. Ovisacs less than -g^-atli °f an inch in diameter and without ducts. 
Pig. 1, PI. XXXI. , represents an ovisac measuring ^oth of an inch in diameter, 
ellipsoidal in form, and nowhere presents any prolongation which can be regarded 
It 
udiment of a duct. The wall of the ovisac is comparatively thick, and obscurely 
