M. Van Beneden on the Physiology of the Simple Ascidians. 251 



to explain these singular embryogenic phsenomena, 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 AscidiiE reproduce themselves by bud and by egg, 

 and the embryo runs through different phases ; — if it is born of 

 an e^^y 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 Ascidice, 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 itseK into several germs which remain grouped in a deter- 

 minate order. That little colony, formed at first by the reunion 

 of several germs natm'ally fissured, may root itself upon a soHd 

 body, and never again leave this resting-place. Or again, 3. as 

 in the PyrosomcBf the colony may remain afloat and continue to 

 swim freely in the bosom of the waters. 4. The Salpce offer still 

 another phsenomenon : there are Salpce which live in isolation and 

 free, and there are Salpce which form long chains composed of 

 individuals joined together. We see in this peculiarity, which has 

 been explained indifferent ways, nothing more than a phsenomenon 

 exactly alike to that which the Ascidice have shown us. Whether 

 it is the effect of age or not, this always holds, that it is the same 

 species which presents the phsenomenon of the free and separate 

 life and of the aggregated or combined life. . The interpretation 

 of the latter phsenomenon 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 theoiy 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 phsenomenon appears at a 



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