90 SURVEY OF THE REPRODUCTIVE PROCESS 



in Salpa (and possibly in other genera) there is an alterna- 

 tion of two forms, from the sexual organs making their ap- 

 pearance only in those zooids which are derived from the 

 original form by gemmation. It was the discovery of the 

 connection between the two forms of Salj^oe that first led 

 Chamisso to introduce the expression of '' Alternation of 

 Generations ;" a term since extended by Steenstrup and 

 others to a variety of cases — some of them of very doubtful 

 relationship. The chains which the aggTegated Salpss 

 form by the cohesion of processes of their gelatinous integu- 

 ment, are modelled within the respiratory sac of the parent, 

 on a j)eculiar tube, which may be compared to an introver- 

 sion of the external stolon of other Tunicata. The tube, 

 being an outgrowth from the vascular system, serves to 

 establish a continuity between the circulation of the parent 

 and that developed in each of the gemmae on its exterior. 

 The tube appears to be in a state of continuous growth 

 from its attached extremity, for the gemmae are most ad- 

 vanced at its free end. As they attain maturity, a portion 

 of the tube breaks off from time to time with its encrusta- 

 tion of gemmae, and escapes with the expiratory current, as 

 a freely floating Salpa-chain. The aggregated Salpce are 

 bisexual, though, as the ova are much in advance of the 

 spermatozoa, they cannot be self-impregnating, but must 

 depend for fecundation on the entrance of the spermatic 

 particles of other chains with the water of the respiratory 

 current, for the development of the contained ova is well 

 advanced before the catenated gemmae are themselves throT\^i 

 off. Each one of the Salpa? composing a chain matures, by 

 the ordinary process of development, a single embryo, which 

 eventually becomes a Salpa of the solitary kind, and in turn 

 buds off in its OAvn interior other chains of zooids like its 

 o^\Ti progenitors. From the great transparency of these 

 animals, both forms may readily be seen at once — one 

 within the other — and even the rudimentary beginning of a 



