ON THE ANATOMY AND DEVELOPMENT OF PYROSOMA — 349 
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 comparatively 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 con- 
siderably 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 Gamogenesis, or Sexual Development, of PYROSOMA GIGAN- 
TEUM (Plate XX XI. [Plate 30]). 
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 development 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. [Plate 29] fig. 23. 
First Stage. Ovésacs less than ziyth of an inch tu diameter and 
without ducts. 
Fig. 1, Pl. XX XI. [Plate 30], represents an ovisac measuring 735th 
of an inch in diameter. It is ellipsoidal in form, and nowhere presents 
any prolongation which can be regarded as even the rudiment of a 
duct. The wall of the ovisac is comparatively thick, and obscurely 
cellular in structure, but it is devoid of any structureless investment or 
membrana propria. The contained ovum consists of a solid-looking, 
well-defined germinal spot z75 th of an inch in diameter, occupying 
the centre of a germinal vesicle ;ijth of an inch in diameter, with a 
thin but well-defined wall, and perfectly clear contents. The yelk is 
represented by a small zone of structureless, yellowish substance, 
which invests the germinal vesicle, and, on the one hand, passes into 
the wall of the ovisac, while, on the other, it is separated from that 
wall by a narrow clear space. 
