M. Van Beneden on the Physiology of the Simple Ascidians. 251 
to explain these singular embryogenic phenomena, by keeping 
simply in view the facts which, within these few years, have been 
added to science. 
The class Tunicata exhibits these modes of reproduction :— 
1. The simple Ascidie reproduce themselves by bud and by egg, 
and the embryo runs through different phases :—if it is born of 
an egg, it will be nomade in its youth, and its figure will be that 
of a tadpole ;—if, on the contrary, it proceed from a bud, the 
embryo will attain its adult character more quickly and by the 
most direct means, without ceasing to remain united to its colony, 
and without presenting any great external changes. 2. The 
compound Ascidia, all reproduced by buds, present nevertheless 
two distinct modes of embryonic evolution after their escape from 
the egg. Instead of undergoing simply its metamorphoses as in 
the preceding instance, the tadpole embryo may spontaneously 
divide itself into several germs which remain grouped in a deter- 
minate order. That little colony, formed at first by the reunion 
of several germs naturally fissured, may root itself upon a solid 
body, and never again leave this resting-place. Or again, 3. as 
in the Pyrosome, the colony may remain afloat and continue to 
swim freely in the bosom of the waters. 4. The Salpe offer still 
another phznomenon : there are Salpe which live in isolation and 
free, and there are Salpe which form long chains composed of 
individuals jomed. together. We see in this peculiarity, which has 
been explained in different ways, nothing more than a phenomenon | 
exactly alike to that which the Ascidze have shown us. Whether 
it is the effect of age or not, this always holds, that it is the same 
species which presents the phenomenon of the free and separate 
life and of the aggregated or combined life. The interpretation 
of the latter phenomenon by Chamisso, generalized by Steen- 
strup, appears to me inadmissible, and little at conformity with the 
great simplicity we everywhere observe to prevail in nature. We 
may from this time forward reduce to a formula the theory of 
embryonic development in the animal scale. 
When on this subject I may be permitted to say a word upon 
that mystery of mysteries—the generation of the Aphides. It is 
known that these insects bring forth young throughout the sum- 
mer without the concourse of males; that eight or nine genera- 
tions in succession are exclusively composed of fruitful females ; 
that all these generations are viviparous, and that at the end there 
is born a generation composed of males and of females ; that then 
there is a coupling, and, instead of producing living young, the 
female now lays her eggs. These are facts, notwithstanding that 
some naturalists still disbelieve in them, and remain unconvinced 
by experiments conducted through years by the most celebrated 
observers. However inexplicable the phenomenon appears at a 
