92 
THE VOYAGE OF H.M.S. CHALLENGEE. 
first appearance of the reproductive organs in the form of a few primitive ova placed 
dorsally and posteriorly, behind the branchial sac. The mass is of rounded form, and 
contains several young ova. (2) The formation of the intestine as an outgrowth 
posteriorly from the large branchial sac (mesenteron). It has the form of a short slightly 
curved blind tube (PI. IX. fig. 9, ^). (3) The union of the two lateral peribranchial 
spaces dorsally to form the future cloaca (PL IX. fig. 10,y>.6r.) Figure 9 represents a 
stage rather more advanced than figure 10. It shows the rectum leading up to the 
peribranchial cavity. 
After this stage a gap occurs. With the exception of one specimen showing a 
transverse section through a branchial sac (PI. IX. fig. 11) in which two rows of stigmata 
are already formed on each side and in which the endostyle is seen in the form of a pair 
of parallel folds, there are no further specimens till a stage is reached so far advanced 
that it can hardly be called a bud. It is a young Ascidiozooid (PI. IX. fig. 12) with 
all its organs and four rows of roundish stigmata already formed. Its body is obscurely 
divided into the two regions of the adult Ascidiozooid, and, from the posterior end of 
the abdomen, the vascular appendage has begun to bud out. The position and destiny 
of this young Ascidiozooid will be discussed presently. 
The preceding outline of the j)rocess of gemmation shows several points of interest, 
the first of which is the important j30sition occupied by the vascular system in the 
formation of buds. In tins respect, however, the present species does not stand alone, 
but resembles the Clavelinidse amongst Ascidiee Simplices and Didemnium styliferum 
(Kowalevsky), Sarcohotrylloides wyvillii ixud. some other forms in the Ascidiae Compositse. 
The division of the archenteron into three longitudinal cavities, the two lateral being 
destined to form by their union dorsally the peribranchial cavity of the adult, seems to 
be a phenomenon of very general .occurrence in Ascidian buds, and was described long 
ago by Metschnikoff in Botryllus and by Kowalevsky in Perophora, Didemnium, and 
Amaroucium. No transverse division of the buds such as takes place in Botryllus 
(Metschnikoff and Krohn), Didemnium (Kowalevsky), and Amaroucium (Kowalevsky) is 
met with in the present species. 
In this case, as in others (see p. 59), I believe that it is possible to trace back the 
structures which enter into the formation of the bud to the two primary getm layers of 
the body of the adult Ascidiozooid. The wall of the vascular appendage which probably 
forms the outer layer of the bud is continuous with the mantle, and is therefore covered 
by ectodermal cells. The blood-corpuscles which lie in the vascular appendage and form 
the inner layer of the young bud are mesodermal cells. Now it has been shown by 
Kowalevsky and by E. van Beneden that in various Ascidians the mesoblast in the 
cmbr}m is derived from the hypoblast.’^ Consequently the mesoblast cells, which become 
^ See also Seeli^er, Ziir Ent\vickelun"sgescliiclite der Ascidien, Sitzunqsb. d. h. k. Akad. d. TViss. Wien, Ed. Ixxxv. 
Ilelt iv. p. .3G4, 1882. 
