1876.] A Remarkable Life History and its Meaning. 655 
than a fiftieth of its adult size, its egg is fertilized. Going back 
to a still earlier period we find that when the organs of the 
chain-salpa first begin to make their appearance upon the walls 
of the tube, each one contains a full-grown egg, as shown at s in 
i 58. At a still earlier period, when the only indication of 
the future chain-salpa is the constriction upon the surface of the 
tube, each space thus marked off contains a single, full-grown 
egg, which appears to be as fully ripe as at the time of impreg- 
nation. Figure 59 shows five of these constrictions at this time, 
and their eggs (s). 
Ata still earlier stage, before the constrictions appear upon 
the wall of the tube, this is seen to contain two large club-shaped 
bodies (Figure 47, yy), which under careful examination with 
high powers are found to contain germinative vesicles ; and by 
patient examination of large numbers of solitary Salpz at about 
this period, a few may be found which show that these bodies are 
made up of rows of eggs and are therefore ovaries, and the soli- 
tary Salpa must be regarded as the female, since the chain-salpa 
cannot be the parent of an egg which exists before the chain- 
salpa itself is formed. 
We must therefore conclude that, instead of an instance of 
“alternation of generations,” we here have simply a remarkable 
difference in the form and mode of origin of the two sexes, for 
We must regard the solitary Salpa as the female and the chain- 
salpa as the male. The life-history of Salpa may then be briefly 
stated in outline as follows: The solitary Salpa is the female, 
and produces a chain of males by budding, and discharges a sin- 
gle egg into the body of each of these before birth. These eggs 
are impregnated while the chain-salpe are very small and sexu- 
y immature, and develop into females which give rise to males 
by budding. After the foetus has been discharged from the body 
_ of the male the latter attains its full size, becomes sexually ma 
. tare, and discharges its spermatic fluid into the water to gain ac- 
ess to the eggs carried by other immature chains. | 
It is worthy of notice that although Chamisso 8$ announcement 
of the occurrence of alternation of generations among animals is 
thus seen to have been drawn from the study of animals which 
do not present an instance of it, this mistake has been of the 
_ SFeatest usefulness, since it has led to our knowledge of the nu- 
= Merous instances of true alternation which now form such a large 
= “Md important chapter of zodlogical science. The relation in 
_ Which Salpa stands to the other tunicates shows also that no 
