AND DEVELOPMENT OF PYROSOMA. . 213 



taper end of this sac seems to be attached to the inner surface of the apex of the outer 

 tunic of the ascidiozooid. Anteriorly and posteriorly, its walls appear thick, the enlarge- 

 ment being much more marked posteriorly. The side of the sac turned towards the eye, 

 between these thickenings, exhibits five faint rings, with comparatively clear centres. In 

 order to avoid circumlocution, I may so far anticipate the results yielded by the investi- 

 gation of later stages of the buds, as to state the nature of the parts which have now 

 been described. The rings are the indications of the commencement of as many bran- 

 chial stigmata ; the anterior apjoarent thickening is the result of the^ formation of the 

 rudiment of the endostyle ; the posterior apparent thickening is produced by the rudiment 

 of all that part of the alimentary tract which lies behind the branchial sac, into which 

 almost the whole of the dilated end of the prolongation of the endostylic cone is converted. 

 A comparatively clear space surrounds the apex of the branchial sac, below which the 

 inner surface of the external tunic presents a band-like aggregation of indifferent tissue, 

 the rudiment of a body which corresponds with what Krohn has called the elseoblast in 

 the SalpcB ; and finally, projecting from the posterior wall of the external tunic, and appa- 

 rently connected with the elgeoblast, is an elongated mass, the anterior portion of the gene- 

 rative blastema, which has now become completely separated from the posterior part. The 

 anterior end of the latter, in fact, extends only into the peduncle, while its posterior moiety 

 lies, attached to the outer tunic of the parent, in the great hsemal blood-sinus.. The genera- 

 tive blastema may therefore be now distinguished into three parts — parental, peduncular, 

 and gemmular, — of which the two former remain connected, until a new bud is developed 

 in the distal end of the peduncle, while the latter, now contained wholly within the bud, 

 and separated from the others by a considerable interval, is itself divisible into three 

 portions. The first of these is the ovum, or rather ovisac, larger and more distinct 

 than in the last- mentioned stage ; the second is that part of the blastema near this, 

 which will become the testis, but which, at present, has no definite form ; and the third 

 is represented by a slender band of indifferent tissue continued up to the apex of the 

 branchial sac (the future extremity of the endostylic cone) which is the generative blastema 

 of the nascent ascidiozooid and Avill supply reproductive organs to its buds. The interspace 

 between the branchial sac and the outer tunic of the bud is in free communication with 

 the blood-sinus of the parent, by means of the interval between the endostylic cone and the 

 wall of the peduncle ; and, in fact, this interspace is itself the foundation of all the blood- 

 sinuses of the bud. 



This is as much as can be clearly made out from the inspection of side views of buds 

 in this and earlier stages ; but much additional information is to be gained from other 

 views of similar buds. Fig. 26 exemplifies the appearances yielded by a bud about t9tIi<^ 

 of an inch in diameter, when seen from above. 



The transverse section of the wide sac-like prolongation of the endostyle thus presented 

 to the eye is four-sided ; the lateral and posterior walls are concave, while the middle of 

 the anterior wall is produced into a sort of fold ; so that the contour of the sac may be 

 compared to that of a crown. Masses of indifferent tissue fill the interspace betAveen the 

 concavity of the lateral wall of the sac and the sides of the external tunic, while the in- 

 terval between the posterior wall and the hinder part of the external tunic is nearly filled 



