124 The Science of Life. 



made a casual observation which has since become very 

 famous. He observed that in a species of the free- 

 Alternation swimming' Tunicate, Salpa, a solitary form 

 of Genera- gave rise to embryos quite different in char- 

 lons * acter and linked together in a chain, and 



that each link of the chain again produced a solitary 

 form. His observation was not altogether accurate, 

 but it called attention to a remarkable fact, which for a 

 time seemed to stand alone. 



The progress of marine zoology and the study of 

 parasites, in the hands of men like Sars, Dalyell, Lov6n, 

 Von Siebold, and Leuckart, disclosed other alternations 

 somewhat similar to that observed by Chamisso, but 

 the results were not generalized until 1842, when Steen- 

 strup (1813-1897) published a work entitled, On the 

 Alternation of Generations; or, The Propagation and 

 Development of Animals through alternate generations, a 

 peculiar form of fostering the young in the lower classes of 

 animals. From Hydroids (zoophytes) and Trematodes 

 (flukes) he gave illustrations of the "natural pheno- 

 menon of an animal producing an offspring which at 

 no time resembles its parent, but which itself brings 

 forth a progeny that returns in its form and nature to 

 the parent ". 



In 1838-39, as we have already noticed, Schwann and 

 Schleiden formulated the cell-theory, towards which the 

 The influence resear "ches of many workers had been steadily 

 of the ceil- leading. In this doctrine there were three 

 correlated conclusions: (a) that the organism 

 has a cellular structure ; (b) that its life depends on the 

 reciprocal action of the component cells; and (c) that 

 development means cell-formation, and begins by the 

 cleavage of the ovum. "Every elementary part", 

 Schwann said, "possesses a power of its own, an inde- 

 pendent life, by means of which it would be enabled to 

 develop independently, if the relations which it bore to 

 external parts were but similar to those in which it 

 stands in the organism. The ova of animals afford us 

 examples of such independent cells, growing apart from 

 the organism." Under the influence of the cell-theory 

 it became the pressing task of the embryologists to 



