28 APPENDICULARIA. 



Development of the chain of sexual Salps. My description of 



the embryonic development of Salpa would not be complete without some 

 reference to the development of the stolon of the solitary generation of 

 Salps by the segmentation of which a chain of sexual Salps originates. 



The asexual Salp, the embryonic development of which has just been 

 described, may be compared to the Cyathozooid of Pyrosoma, from which 

 it mainly differs in being fully developed. While still in an embryonic 

 condition it gives rise to a process or stolon, which becomes divided into a 

 number of zooids by transverse constrictions, in the same manner that part 

 of the germ of the ovum of Pyrosoma is divided by transverse constrictions 

 into four Ascidiozooids. 



The stolon arises as a projection on the right side of the body of the 

 embryo close to the heart. It is formed {Salensky, No. 35) of an outgrowth 

 of the body wall, into which there gi'ow the following structures : 



(1) A central hollow process from the end of the respiratory sack. 



(2) A right and left lateral prolongation of the pericardial cavity. 



(3) A solid process of cells on the ventral side derived from the same 

 mass of the cells as the elteoblast. 



(4) A ventral and a dorsal blood sinus. 



Besides these parts there appears on the dorsal side a hollow tube, the 

 origin of which is unknown, which gives rise to the nervous system. 



The hollow pi-ocess of the respiratory sack is purely provisional, and 

 disa[)pears without giving rise to any permanent structure. The right and 

 left prolongations of the pericardial cavity become solid and eventually 

 give origin to the mesoblast. The ventral process of cells is the most 

 important structure in the stolon in that it gives rise both to the alimentary 

 and respiratory sacks, and to the genei'ative organs of the sexual Salps. 

 The stolon containing the organs just enumei'ated becomes divided by 

 transverse constrictions into a number of rings. These rings do not long 

 remain complete, but become interrupted dorsally and ventrally. The imper- 

 fect rings so formed soon overlap, and each of them eventually gives rise to a 

 sexual Salp. Although the stolon arises while the asexual Salp is still in 

 an embryonic condition, it does not become fu.lly developed till long after 

 the asexual Salp has attained maturity. 



Appendicularia. Our only knowledge of the development of Appen- 

 dicularia is derived from Fol's memoir on the group (No. 8). He simply 

 states that it develops, as far as he was able to follow, like other Ascidians ; 

 and that the extremely minute size of the e^g prevented him from pursuing 

 the subject. He also states that the pair of pores leading from the bran- 

 chial cavity to the exterior is developed from epiblastic involutions meeting 

 outerrowths of the wall of the branchial sack. 



Metagenesis. 



One of the most remarkable phenomena in connection with the 

 life liistory of many Ascidians is the occurrence of an alternation of 

 sexual and gemmiparous generations. This alternation appears to 

 have originated from a complication of the process of reproduction by 

 budding, which is so common in this group. The mode in which this 

 very probably took place will be best understood by tracing a series 



