THE BUDDING AXD FISSION OF ASCIDIANS. 525 



{u') has already taken on the character of an ovum, and is 

 surrounded by a rudimental ovisac. The conical bud elongates 

 and dilates at its extremity, and the dilatation gradually takes 

 on the form of a new zooid united by a narrow neck, or pe- 

 duncle, with the parent (Fig. 150, IV.). The endostylic cone 

 gives rise to the whole alimentary canal of the bud, while the 

 ectoderm of the latter proceeds from the ectoderm, and its 

 ovisac and testis from the mesoblastic cells, of the parent. 

 Thus the organs of the bud are all the direct product of the 

 corresponding parts, or of the primitive layers of the germ 

 from which they are derived, in the parent.* 



After the terminal bud is formed, a second is usually de- 

 veloped immediately below it (Fig. 150, VI.) by the growth 

 of the ectoderm, endodermal axis, and mesoblastic cells of the 

 peduncle ; and it would appear that this process is frequently 

 repeated. The fully-formed bud becomes detached, and takes 

 its place among the other zooids in the test, there to repeat 

 the process of gemmation. 



The observations of Krolin, MetschnikofF, and Kowalewsky, 

 have shown that two components enter into the buds of ascid- 

 ians in general ; first, an outer la3'er consisting of the ecto- 

 derm of the region in which the budding takes place, and, 

 secondly, an inner layer derived from the endoderm of the 

 branchial sac (Perop/iora) ; or, as in J^otryllus^ according to 

 MetschnikofF, from the atrial tunic' To these must be added 



^ In my second memoir on Pyrosoma C' Trans. Linn, Society," xxiii., p. 

 211) I have said: 



" Gemmation does not take place in Pyrosoma as in so many of the lower 

 animals (e. g., the Bydrozoa and Polyzoa^ or Salpa and Clavelina, amonsr the 

 ascidians), by the outgrowth of a process of the body-wall, whose primarily "whol- 

 ly indifferent parietes become differentiated into the organs of the bud; but, 

 from the lirst, several components, derived from as many distinct parts of the 

 parental organism, are distinsruishable in it, and each component is the source 

 of certain parts of the new being, and of them only. Thus the body-wall or 

 external tunic of the parent gives rise to the external tunic of the bud ; while a 

 process of the endostvlic cone of the parent is converted into the alimentary 

 tract of the bud, and the reproductive organs of the latter are furnished by a 

 part of that tissue whence the reproductive organs of the parent took their 

 origin." 



As will appear further on. however, recent investisrations show that the 

 •whole process of budding in the srreat majority of the Tf/nicata, and at any rate 

 the first steps of that process in Salpa, are essentially similar to those in Pyro- 

 soma ; and it remains to be seen whether there is any difference in other As- 

 cidians. And as resrards even the ffydrozoa, the expression that the parietes 

 of a bud are at first " wholly indifferent " in structure is not quite accurate, in- 

 asmuch as they are composed of an ectodermal and an endodermal layer, which 

 are continuous with those of the parent, and give rise to homologous orsrans. 



^ If, as some observations tend to show, the atrial tunic itself is a diver- 

 ticulum of the primitive endoderm, this case would form no exception to the 

 general law of budding in the Tunicata. 



