232 SUMMARY OF CURRENT RESEARCIIES RELATING TO 



growth from tlic dorsal side of the nurse, and to the ventral outgrowth 

 of the foster-animal. The development of the three different forms of 

 the sexual generation from the buds, and the structure of the nutrient 

 and foster-animals, are described in the fifth chapter. In the sixtli, 

 the first point discussed is the relation of the reproductive pro- 

 cesses of Doliolum to those of other Tunicates; Uljanin believes that 

 he has demonstrated that tlie alternations of generation in Doliolum 

 are confined to two generations, and that the process is much simpler 

 than has been generally supposed. There can be no doubt that the 

 Dolicltim-nnrse corresponds to the nurse of other alternately generating 

 Tunicata ; like the soUtarj Saltxi and the cyathozooid of Pijrosoma, it 

 is provided with a stolo prolifer ; the sexual genci-ation clearly cor- 

 responds to the ascidozooids of Pyrosoma and the chain-form of the 

 Salpidae. But it is essentially distinguished by its polymorphism, 

 which appears to bo unique among the Tunicata. Its relations to 

 Anchinia, Distaplia, and Didemnhim, are afterwards considered. 



The next question which arises is as to the origin of metagenesis 

 among the Tunicata. The Appendiculata appear to be the oldest 

 members of the group, and, like the Vertebrata to which they stand 

 nearest, they are not metagenetic ; the origin of the phenomenon must 

 be sought for in the division of extremely early stages of development, 

 in which the tissues were very slightly diiferentiated ; this being of 

 advantage, might in time come to be constant ; later it would be 

 converted into budding and carried on to somewhat later stages of 

 development ; this view is rendered the more probable by the observa- 

 tion of Eauber that divisions sometimes occur in vertebrate embryos, 

 where they are of course only exceptional, and only lead to tcrato- 

 logical cases, and is supported by the fact that asexual multiplication 

 occurs at a very early stage in the development of Tunicates. The 

 prolongation of the period of formation of the stolo prolifer led to the 

 institution of the asexual and independent being which is known as 

 the nurse ; and this is very variously constituted in various Tunicates. 

 The author gives a table to show the advantage to the species of the 

 formation of buds, from which we learn that while in the simple 

 Ascidians, which are exclusively produced by sexual means, sixty-four 

 individuals come from one egg by the fourth generation, the Cyclo- 

 myaria, whose larvae are converted into nurses, give rise by the fourth 

 generation to 16,384 individuals. By the fourth generation of Bo- 

 tryllus one egg has given rise to six millions of individuals. What 

 we know of the mode of reproduction of the Tunicata leads us to 

 suppose that there is not in all cases a regular alternation of genera- 

 tions, but that agamic reproduction is sometimes combined with sexual. 



The second or systematic part of the monograph first discusses 

 the genealogy of the Tunicata ; the author inclines to the view that 

 they re2)resent a side-branch of the vertebrate phylum, whose point 

 of origin is near its root. As already said, the Appendicularia are 

 regarded as the most primitive rejiresentatives of the gi'oup ; they 

 gave rise to the simple Ascidians ; thence there branched off the 

 Salpidie on the one hand, and the compound Ascidians on the other ; 

 the latter gave rise to the social Ascidians, to Botryllus, and to Pyro- 



