ALTERNATION OF GENEBATIONS IN THE TUNICATES. 515 



reason why we should not assume that the capacity for division was 



not possessed by the adults from the first, indeed, this capacity may 

 possibly have been inherited directly from older pelagic ancestors 

 of the Tunicates. For, even if the circumstance that the Larvacea 

 do not multiply asexually seems to indieate that this form of re- 

 production was acquired only after attachment, we are not sure that 

 this was the ease. 



Seeliger, who attributes to the mesoderm the principal part in 

 the development of the proliferating stolon, and who derives the 

 mesoderm of the stolon, at any rate in Pyrosoma, from the genital 

 tissues of the parent, finds in the limitation of sexual reproduction 

 the cause for the development of buds, the material left in the 

 ovary after the production of a single egg is utilised in its plastic 

 capacity as the mesoderm of the stolon. But even if we make this 

 assumption, the manner in which budding was acquired remains 

 obscure. 



Further confusion has been introduced into the views as to the 

 alternation of generations in the Tunicates by the fact that the egg 

 cells frequently maturate very early in the buds. They can even be 

 distinguished in the genital strand of the stolon in Pyrosoma and 

 Salpa. This has, in many cases, led to the view that the ovary 

 actually belongs to the solitary form and is merely deposited in the 

 forms composing the chain. This view, which is adopted by Brooks 

 (Nos. 88-91) does not seem justifiable to us. We prefer Seeliger's 

 view that the egg with its follicle is just as much an organ of 

 the individual of the chain as are all the other organs. The early 

 differentiation of the egg-cells is to be traced back to the effort on 

 the part of the organism to arrive at sexual maturity as early as 

 possible. The same hastening of sexual maturity is found in the 

 Hydroids and, indeed, in the parthenogenetic CladoCera and Aphidae 

 and also in the Diptera (polar cells). 



P>kooks regards all the ovaries of the individuals of a Salpa-chain taken 

 together as the germ-gland of the solitary form shifted into the stolon. He 

 considers the solitary forms not as sexless but as females, while he regards the 

 individuals of the chain as males which have arisen from the females as buds. 

 The solitary form deposits an egg in each male, and this, when fertilised, 

 deveiup-^ into a female. Brooks therefore reduces the alternation of genera- 

 tions of Salpa to a kind of sexual dimorphism. 



We have already stated (p. 496) that Todaro, by deriving all the buds from 

 certain embryonic germ-cells (germoblasts), and by tracing these latter directly 

 to the blastomeres of the embryo, regards the individuals of a Salpa-chain not 

 as the descendants of the solitary form hut merely as younger members of the 



