298 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



equal in number to the future salpae, and not twice as numerous. As 

 each salpa is constricted off from the tube, it carries with it the greater 

 part of one of the masses of cells from one side of the stolon, and the 

 lesser portion of the one on the opposite side. These two masses are not 

 bilaterally placed in the body, but are on the middle line, the larger one 

 being dorsal or neural, and the smaller one ventral or haemal. The latter 

 gives rise to the heart and to the eleoblast, while the larger one on the 

 neural surface gives rise to most of the mesoderm of the chain-salpa, and 

 also to a cloacal vesicle, which is median and unpaired. This vesicle 

 becomes distended, and at two points, one on each side of the middle line, 

 it unites with the wall of the branchial sac, and the atrium and the 

 branchial chamber thus become connected through the two gill-slits, 

 while a similar union with the ectoderm on the dorsal middle line forms 

 the atrial aperture. 



Seeliger's account and figures show his persistency, and his account 

 is as near to the truth as one could hope to get by the study of trans- 

 verse sections of the stolon of Salpa democratica, but a very little study 

 of sections in other planes, in more favorable species, will show that he 

 has completely failed to understand the history, and that his account 

 has little permanent value. 



It is not only irreconcilable with my own observations, but also with 

 our knowledge of pyrosoma, for both Seeliger (15, pp. 622-624) and Salen- 

 sky (17, pp. 31-36) state that, in this genus, the perithoracic system is 

 bilaterally symmetrical; that each bud has two perithoracic vesicles 

 which are not dorsal and ventral, but right and left ; that each of them 

 unites with its corresponding half of the pharynx to form the gill-slits 

 before the two vesicles unite with each other to form the median atrium, 

 but that this arises, as it does in the aggregated salpa, on the dorsal 

 middle line by the meeting and union of diverticula from the two 

 vesicles, and that the external aperture arises still later, as it does in 

 salpa, as an independent aperture on the middle line. 



The perithoracic vesicles are derived, as they are in salpa, from 

 the right and left perithoracic tubes of the stolon, but, in the primary 

 ascidiozooids at least, these are continuous with the perithoracic tubes 

 of the primary embryo or cyathozooid, where, according to both Kowa- 

 levsky (Arch, f . mik. Anat. XI, 1875) and Salensky (17, pp. 466, 473, 475), 

 the evidence that they arise as paired ectodermal invaginations from the 

 surface of the body is clear and unmistakable. 



