ANATOMY AND PHYSIOLOGY OF SALPA AND PYROSOMA 5 I 



35. It is not clear by what channel fecundation takes place, whether 

 each Salpa B impregnates its own ovum by discharging the contents 

 of its own testis into the circulatory fluid, which would be a procedure 

 altogether anomalous ; or whether, on the other hand, impregnation do 

 not rather take place from without, a presumption which is strengthened 

 by analogy, and by the fact, that the testis does not seem to attain 

 maturity early enough to fecundate its own ovum. The spermatic 

 fluid may have access to the ovum by the gubernaculum becoming 

 hollow and tubular, as will be seen to be the case in the Pyrosoinata, 

 and indications of such an occurrence have occasionally manifested 

 themselves. 



36. To recapitulate. — The form A {Salpa solitaria) produces a 

 stolon, from which, by gemmation, arises the form B {Salpa gregata). 

 This contains a testis and a single ovum attached by a pedicle or 

 " gubernaculum " to the wall of the respiratory chamber. Fecunda- 

 tion takes place in a manner not yet clearly ascertained, and the 

 " gubernaculum " shortens until the ovum is brought into close contact 

 with the respiratory wall or inner tunic. The latter then protrudes 

 into the respiratory canal, enveloping the ovum in a close sac ; the 

 ovum becomes developed into an embryo, which is connected by a 

 genuine placenta with its parent, and ultimately assuming the form of 

 Salpa A becomes detached and free. 



37. While Chamisso's formula, then, expressed the truth with 

 regard to the generation of the SalpcB, it does not express the whole 

 truth. 



True it is, that the Salpa solitaria always produces the Salpa gregata, 

 and the Salpa gregata the Salpa solitaria ; but it is most important to re- 

 member that the word " produce '' here means something very different 

 in the one case, from what it means in the other. In the Salpa 

 solitaria the thing produced is a bud ; in the Salpa gregata a true 

 embryo. There is no " alternation of generations," if by generation 

 sexual generation be meant ; but there is an alternation of true sexual 

 generation with the altogether distinct process of gemmation. 



It would be irrelevant to discuss here the wide question of the 

 " alternation of generations " in all its bearings ; but the writer may be 

 permitted to express his belief, founded upon many observations 

 upon the Polypes, Acalephs, &c., that the phenomena classed under 

 this name are always of the same nature as in the SalpcB; that under 

 no circumstances are two forms alternately developed by sexual gene- 

 ration ; but that wherever the so-called " alternation of generations " 

 occurs it is an alternation of genei'aiion ivitli gemmation. 



38. Using the terminology of insect metamorphosis, as Chamisso 



I-; 2 



