24 
THE VOYAGE OF H.M.S. CHALLENGER. 
Usually the embryos undergo their entire development in the body of the parent, 
either in the peribranchial cavity or in a special incubatory pouch, and do not pass into 
the outer world until they have become completely developed tailed larvae. 
Gemmation. 
The budding of Compound Ascidians is a very important process — (l) because 
it is a very common method of reproduction with most species, and (2) because 
it has a most important bearing upon the characteristics of the colony. 
Eeproduction by gemmation takes place in a number of difterent ways amongst the 
Ascidise Compositse, in fact nearly every species in which the process has been 
carefully examined has been found to have a method more or less peculiar to itself. 
Most of these methods, however, fall into one or other of a few main types of 
budding, and in all cases the process may be considered as consisting of the giving off 
from the parent Ascidiozooid of a number of cells containing representatives of the 
three primary layers of its body, ectoderm, mesoderm, and endoderm. 
Giard in 1872^ recognised four distinct methods of budding which he called 
stolonial, pallial, ovarian, and pyloric, and as these names are useful in indicating the 
positions of the buds, they may be retained with advantage. 
Stolonial is the process seen typically in the Clavelinidse amongst Simple Ascidians, 
but found also sometimes in the Botryllidse (see p. 59). Here the bud is formed from 
the enlarged knobs upon the so-called “ vessels ” or stolons, which are really prolonga- 
tions of the ectoderm and mantle of the posterior part of the body, and contain 
prolongations of the vascular sinuses of the mantle (see fig. 2, p. 14). 
Pallial budding is seen also in the Botryllidse, as was shown long ago by Krohn and 
by Metschnikoff, and it is by this process that the systems in the colony are mainly 
formed. The bud is produced as a lateral outgrowth from the body of the parent 
Ascidiozooid. 
Ovarian budding is found in the Polyclinidae, where the reproductive organs 
extend behind the alimentary canal to form what is called the post-abdomen. 
This region of the body gives rise to buds either by breaking up into a number of 
pieces or by giving off processes. This method of budding is really the same as the 
stolonial. In the first place there are several cases known which seem to be ipter- 
mediate between the two, and where it is impossible to say definitely which process is 
followed ; and secondly, the post-abdomen is simply a vascular appendage or stolon 
which has become in great part filled up by the large reproductive organs of the 
Polyclinidae, and the process given off from the post-abdomen has exactly the same 
structure as the knob upon one of the vessels of the Botryllidse. 
* Recherches sur les Synascidies, Archives d. Zool. expdr., vol. i. p. 570. 
