AND DEVELOPMENT OF PYROSOMA. 219 



end of the saccular rudiment of the alimentary tract (figs. 15 and 16), and seems to be the 

 means of connecting the end of that sac with the external tunic. 



After a time, however, a clear space appears around the apex of the sac, and separates 

 this connecting mass from the rest, which now, consequently, appears as a broad zone («) 

 surrounding the sac below its apex, but above the uppermost of the branchial stigmata 

 (fig. 19). This zone remains as a broad, thickish girdle of indifferent tissue, closely con- 

 nected with the outer tunic externally and in front, and with the generative blastema 

 behind, in buds of -T^^tli — -^oth of an inch long (figs. 21 and 22) ; but in larger zooids its 

 tissue has undergone a great change, and it has become a transparent mass, through 

 which ramified corpuscles, like connective-tissue corpuscles, appear scattered (fig. 23). In 

 this condition it is exactly analogous to the structure termed elgeoblast in the SalpcB by 

 Krohn. Its bulk is now equal to a fifth or a sixth of that of the entire bud, but in subse- 

 quent stages (figs. 24, 25) it diminishes both absolutely and relatively in size and eventu- 

 ally it disappears. 



In buds sijth of an inch in diameter, the generative blastema remains in its primitive 

 condition, except that it and the ovisac it contains, have increased in size. Its anterior 

 pointed end is closely juxtaposed to the endostylic cone. In the zooid represented in 

 fig. 23, which measured g-Q-th of an inch in length, the generative blastema has become 

 divided into two parts, the smaller of which remains in close apposition to the endo- 

 stylic cone, while the larger, retaining its connexion with the posterior and upper wall 

 of the mid-atrium, becomes widely separated from the other. The interval between the 

 two is occupied by the el^oblast. Even before the separation has taken place, the larger 

 portion has become distinctly differentiated into two parts, the ovisac, on the left, sepa- 

 rating itself from a rounded mass of indifferent tissue, on the right. This last is the 

 rudiment of the testis. Erom rounded, it becomes pyriform, the narrowed neck of the 

 pear remaining in connexion with the atrial wall, and eventually becoming metamorphosed 

 into the vas deferens, while the broad end increases in size, and is directed more forwards 

 as well as upwards. 



In a bud iVth of an inch long (fig. 24) the testis measures -g-s o^h of an inch in length, 

 while its broad end is above -s^o^h of an inch thick. The apex of the vas deferens already 

 pushes a little eminence of the atrial tunic before it. 



In a young ascidiozooid, somewhat more advanced than that represented in fig. 25, 

 the vas deferens is sioth of an inch in length, and is of nearly even diameter throughout, 

 except at its vipper end, where it is slightly dilated and plainly hollow. It is connected 

 with the posterior part of the terminal enlargement, which is nearly ^^oth of an inch 

 thick, and is divided into three short lobes, each about s^^njth of an inch thick. Like 

 the previously existing pyriform enlargement, these rudimentary cpeca are solid masses of 

 indifferent tissue. Traces of a distinct membrana propria are discernible around each 

 CEecum. In still larger ascidiozooids the number of cseca increases, and the whole organ 

 becomes larger, until it assumes its adult form ; and it is only \\\\en nearly in this con- 

 dition, that spermatozoa are visible in the vas deferens and the adjacent parts of the caeca. 



The development of the ovisac will be described below. At first both the testis and 



2g2 



