REPORT ON THE TUNICATA. 
25 
The pyloric method of budding is seen in the Didemnidse, and the buds are formed 
as enlargements upon the ends of long tubular projections from the body- wall in the 
region where the thorax joins the abdomen. Such projections have precisely the 
same structure as the vessels in the Botryllidae, but of course differ from them in 
position. 
In the Diplosomidse, and some Didemnidse, an interesting modification of pyloric 
budding is found, in which the new Ascidiozooid is formed of two buds ^ from the 
parent body, which give rise to the thorax and the abdomen respectively, and only unite 
late on in their development. Delia Valle ^ has added considerably to our knowledge of 
this method of gemmation, and has shown that the thoracic bud is derived from the 
wall of the peribranchial cavity of the parent, while the abdominal bud is formed as 
a diverticulum from the oesophagus. 
This author states that in the Botryllidae buds are only formed from the parietal 
layer of the peribranchial membrane, and are never formed in connection with the 
vascular prolongations of the ectoderm as had been declared by previous investigators. 
I have been able to satisfy myself, however, that Della Valle is mistaken on this point, 
and that, in the case of one species at least [Sarcobotrylloides ivyvillii, see p. 59), buds 
are formed in the dilatations on the vessels of the test (see PI. IV. fig. 13), and therefore 
probably the observations of Milne-Edwards, Giard, and others, in which the “ marginal 
tubes ” were described as being connected with reproduction by gemmation, were perfectly 
correct. 
Wherever it is formed, I believe that the bud always contains (l) a cellular coating 
derived from the ectoderm of the j^arent ; (2) some mesoderm cells, usually in the form 
of young ova; (3) some endoderm cells, either in the form of a diverticulum from some 
part of the alimentary canal of the parent, or as a group of undifierentiated blood- 
corpuscles, which in the young animal are formed from the hypoblast. The young bud 
soon takes the form of two concentric cellular layers, the primitive ectoderm and 
endoderm, with a few mesoderm cells between. The endodermal sac then becomes 
constricted at two points so as to divide its cavity into three sacs, the middle one 
becoming the alimentary canal and the two others joining to form the peribranchial 
cavity of the adult. The primitive alimentary canal soon divides into a larger anterior 
part, the future pharynx or branchial sac and a smaller posterior region, the future 
stomach and intestine. The stigmata form as slits in the double membrane separating 
the lumen of the branchial sac from the peribranchial cavity, the intestine elongates, and 
other organs begin to make their appearance. The further course of the development 
of the bud differs somewhat in the different groups of Compound Ascidians.^ 
1 According to Jourdain’s recent researches, however {Gomptes rendus, t. c. p. 1512, 1885), the two buds arise as a 
single outgrowth from the oesophageal region of the parent Ascidiozooid, and afterwards separate. 
2 Nuovi Contribuzioni alia Storia Naturale delle Ascidie Composte, Beale Accad. dei Lined, Roma, 1881. 
® For further details of the process of gemmation in the genus Colella, see pp. 90 and 101. 
(ZOOL. CHALL. EXP. — PART XXXVIII. — 1886.) Pp t: 
