Embryology of Saljpa. By W. K, Brooks. 
13 
the developmental history of the genus presents a true example of 
" alternation of generations." When, however, we have traced back- 
ward the history of one of the zooids, which compose a chain, and 
find that the egg is present at all stages of growth, and is of exactly 
the same size and appearance as at the time of its impregnation ; 
when we find one organ after another disappearing, until at last 
we have nothing but a faint trace of a constriction indicating upon 
the wall of the stolon the position of the future zooid, the con- 
clusion seems to be irresistible that the animal, which has as yet 
no existence, cannot be the parent of the egg which is already fully 
formed. 
The life history of Salpa may then be stated in outline as follows : 
The solitary Salpa is the female, and produces a chain of males by 
budding, and discharges an egg in the body of each of these before 
birth. These eggs are impregnated while the zooids of the chain 
are very small and sexually immature, and develop into females 
which give rise to other males in the same way. 
After the foetus has been discharged from the body of the male 
the latter attains its full size, becomes sexually mature, and dis- 
charges its spermatic fluid into the water to gain access to the eggs 
carried by other immature chains. 
The fact that impregnation takes place, not, as we might expect, 
within the body of the solitary, but within that of the chain Salpa, 
is no objection to this view, for the number of animals whose eggs are 
fertilized within the body of the female is quite small, and in at least 
one genus. Hippocampus, the eggs are received into a specialized 
brood-sac in the male, and are there impregnated. 
We can also find analogy for the singular fact that the eggs 
always develop females, while the males are formed by budding. 
The fertihzed eggs of the bee always give rise to females, while the 
males are developed by the virgin bee, through what seems, as 
pointed out by Professor McCrady, to be most properly regarded 
as a process of internal gemmation ; and we cannot fail to mark the 
very striking parallelism between the process of reproduction as 
manifested in Salpa and the bee. 
The fertihzation of the eggs within the bodies of zooids produced 
by budding from the body of that whose ovary gave rise to the eggs 
is not unusual among the Tunicata. The zooids of most of the 
Tunicata are hermaphrodite, and develop eggs of their own, but, at 
least in the case of Pyrosoma, Perophora, Didemnium, and Amauri- 
cium, the egg which undergoes impregnation and development within 
the body of the zooid is derived, not from its own ovary, but from 
that of the generation before, and the eggs produced in the body of 
the second generation must pass into the bodies of the zooids of the 
third generation before they can be fertilized. The essential differ- 
ence between this process and that presented by Salpa, is that in 
