578 
MR. HUXLEY ON THE ANATOMY OF SALPA AND PYROSOMA. 
a well-developed testis, rarely contain any embryo, this being generally set free 
when the parent is about half or two-thirds grown. The careful observer will, how- 
ever, be always able to detect a trace of its former attachment, in a sort of cicatrix, 
left at the corresponding part of the respiratory chamber. 
35. It is not clear by what channel fecundation takes place, whether each SalpaB. 
impregnates its own ovum by discharging the contents of its own testis into the cir- 
culatory 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 Pyrosomata, and indications of such an occurrence have 
occasionally manifested themselves. 
36. To recapitulate. — The form A {Salpa soUtaria) produces a stolon, from which, 
by gemmation, arises the form B {Salpa gregata). Tins contains a testis and a 
single ovum attached by a pedicle or “gubernaculum” to the wall of the respiratory 
chamber. Fecundation 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 genera- 
tion of the Salpce, it did not express the whole truth. 
True it is, that the Salpa soUtaria always produces the Salpa gregata, and the 
Salpa gregata the Salpa soUtaria \ but it is most important to remember that the 
word “produce” here means something very different in the one case, from what it 
means in the other. In the Salpa soUtaria the thing produced is a hud-, 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, Acalephae, &c., that the 
phenomena classed under this name are always of the same nature as in the Salpce ; 
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 generation with gemmation. 
38. Using the terminology of insect metamorphosis, as Chamisso has done (70.), 
the larva never produces the imago by sexual generation, the imago again producing 
the larva by sexual generation. But a pseud-imago, which is indeed nothing more. 
