BUDDING IN TUNICATA. 



THE differences between embryogenesis and blasto 

 genesis have often been insisted upon, and in no 

 group more emphatically than in that of the Tunicata. 

 The same or homologous organs have been shown in some 

 cases, and stated in others, to arise in the larva from 

 ectoderm, in the bud from endoderm. It has accordingly 

 been asserted by various writers that the phenomena of 

 budding are at variance with the axioms of the germ-layer 

 theory, and seriously impugn the validity of current ideas 

 upon the differentiation of the germinal layers. This 

 attitude seems to me to be at all events premature. The 

 asserted irregularities of bud development have been cor- 

 rected in more than one important instance ; and, with one 

 exception, the organs whose origin in the bud has been 

 regarded as most inconsistent with the germ-layer theory 

 are precisely those organs concerning the facts of whose 

 development there is most dispute. Difficulties and 

 doubtful points are plentiful enough ; but, with regard to 

 the former, I shall endeavour to show in the present 

 article that they have been exaggerated and are not 

 insuperable ; and, so far as the latter are concerned, it will 

 naturally be necessary for us to await the results of renewed 

 investigations before we can arrive at any decisive con- 

 clusions upon them. I shall be surprised if further research 

 does not continue to remove the apparent discrepancies 

 between bud and larval development, although it is far 

 from my intention to prejudge the important points at 

 issue. It is rather my desire by attempting a critical 

 summary of our existing knowledge to facilitate to some 

 extent the solution of the debated points. 



i. The Participation of the Germ-layers. — In diploblastic 

 animals budding is reduced to its simplest terms. In the 

 calcareous sponges, for example, every stage can be traced 

 between mere sacculation of the body wall — involving out- 

 growths of both germ-layers — and the complete formation 



