GEMMATION 8 3 



formed on a short stolon close to the parent body, or from the 

 extremity of the post-abdomeu (as in the Polyclinidae), or from 

 a long epicardiac tube (as in Colella, Fig. 47), which may extend 

 for some inches from the ascidiozooid. The post-abdomen of the 

 Polyclinidae may be regarded as a stolon invaded by the gonads 

 and the heart (see Fig. 46, C), and traversed by the epicardium 

 in the form of a flattened tube dividing a dorsal blood-sinus con- 

 taining the gonads from a ventral sinus which has merely the one 

 extremity of the tapering pericardium. The whole of this post- 

 abdomen segments to form the buds, the heart at the extremity 

 being absorbed, and a new one formed from the anterior end of 

 the pericardium. 



The epicardium, which supplies the endodermal element to 

 each bud, was first described by E. van Beneden and Julin in the 

 development of Clavelina} as a structure concerned in the forma- 

 tion of the pericardium and heart — hence its unfortunate name. 

 It grows backwards in the larva, from the posterior wall of the 

 branchial sac, close to the endostyle, as a tube which usually 

 divides into two lateral branches to be united again eventually 

 so as to form the single tubular flattened partition of the stolon 

 in Polyclinidae, Distomatidae, Clavelinidae, etc. In some Com- 

 pound Ascidians the epicardium is, from its origin, two distinct 

 lateral tubes, which grow back from the inner vesicle of the 

 embryo (later the branchial sac). These unite in the post- 

 abdomen to form the flattened tube, which in its turn forms the 

 inner vesicle of the future buds, and so the endodermal element 

 is handed on from generation to generation. In addition to the 

 epicardium, the stolon contains also a prolongation of the ovary 

 of the parent, or at least a string of migrating germ ceUs, so 

 that the reproductive elements are also handed on. 



It is clear from the recent researches of Hjort, Eitter, Lefevre,^ 

 and others, that the development of the bud (blastozooid) and 

 that of the embryo (oozooid) do not proceed along parallel lines. 

 It is evidently impossible to harmonise the facts of gemmation 

 with the germ-layer theory ; and attempts to explain budding in 

 Ascidians solely as a process of regeneration by which the organs 

 of the parent or their germ-layers give rise to the corresponding 

 organs in the bud have in many cases failed. 



^ Arch, de Biologic, vi. 1887. 

 ^ See Jowrm. of Morphology, xii.-xiv. 1896-1898. 



