1 62 THE GERM-PLASM 



determinants," and a tliird • blood - corpuscle determinants.'' 

 Various kinds of these cells may easily be distinguished while 

 they still float freely in the blood of the stolon. The difficulty onlv 

 consists in ascertaining the exact part they play in the formation 

 of the developing bud. Those which are to give rise to the 

 longitudinal muscles become arranged in rows, which, diverging 

 oblicjuely from one or two definite points, extend over the animal 

 from behind forwards, and are attached at more or less definitely 

 fixed points anteriorly. The ganglion and the sexual glands 

 have also perfectly definite positions in the animal. In em- 

 bryogeny, as well as in the development of the endodermal 

 vesicle of the bud. the position of every cell is assigned to it 

 mechanically, in consequence of its origin from previous cell- 

 generations, — that is, by the rhythm of the cell-divisions. In 

 the case, however, of the ganglion for instance, the cells of which 

 it is composed must come together at the right place by means of 

 their power of locomotion. A similar process is known to occur 

 in embryogeny in the case of several groups of animals, such as 

 the Ecliinoderjiiata, for instance : and until we know more of 

 the actual facts concerned, we can only — however unsatisfactory 

 such an assumption may be — attribute to the cells a tendency 

 to become attached at definite points according to the manner 

 in which they have previously been determined. The reverse 

 assumption — that these cells develop into muscle-, nerve-, or 

 sexual-cells according to their point of attachment — seems to me 

 at any rate a less likely one. 



If we compare the processes of gemmation and embryogeny 

 in Ascidians, important differences are seen to exist between 

 them. In the former, all the stages of segmentation of 

 the egg and gastrulation, together with the formation of the 

 mesoderm, are omitted : and many parts, again, arise from 

 the ectoderm in the embryo and from the mesoderm in the 

 bud. These differences are perhaps still more marked in the 

 free-swimming Salpce. These animals also multiply by buds 

 produced on a kind of stolon ; and, as in the other Ascidians 

 referred to. the ectoderm forms practically nothing except the 

 epidermis, and the endoderm gives rise to only a few structures, 

 by far the greater number of parts arising from the ' mesoderm- 

 cells.' Seeliger * explains this by supposing that ' the mesoderm 



* Seeliger, ' Die Knospung der Salpen,' Jena, 1885. 



