II GEMMATION 83 
formed on a short stelon close to the parent body, or from the 
extremity of the post-abdomen (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 EK. 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 cells, so 
that the reproductive elements are also handed on. 
It is clear from the recent researches of Hjort, Ritter, 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. 
1 Arch. de Biologie, vi. 1887. 
2 See Journ. of Morphology, xii.-xiv. 1896-1898. 
