UROCHORDA. 29 



of transitional cases between simple budding and complete alternations 

 of generations. 



■ In the simpler cases, which occur in some Composita Sedentaria, 

 the process of budding commences with an outgrowth of the body 

 wall into the common test, containing a prolongation of part of the 

 alimentary tract \ 



Between the epiblastic and hypoblastic layers of the bud so 

 formed, a mesoblastic and sometimes a generative outgrowth of the 

 parent also appears. 



The systems of organs of the bud are developed from the corre- 

 sponding layers to those in the embryo^. The bud eventually becomes 

 detached, and in its turn gives rise to fresh buds. Both the bud and 

 its parent reproduce sexually as well as by budding : the new colonies 

 being derived from sexually produced embryos. 



The next stage of complication is that found in Botryllus (Krohn, 

 Nos. 25 and 26). The larva produced sexually gives rise to a bud 

 from the right side of the body close to the heart. On the bud 

 becoming detached the parent dies away without developing sexual 

 organs. The bud of the second generation gives rise to two buds, 

 a right one and a left one, and like the larva dies without reaching 

 sexual maturity. The buds of the third generation each produce 

 two buds and then suffer the same fate as their parent. 



The buds of the third generation arrange themselves with their 

 cloacal extremities in contact, and in the fourth generation a common 

 cloaca is formed, and so a true radial system of zooids is established; 

 the zooids of which are not however sexual. 



The buds of the fourth generation in their turn produce two or 

 three buds and then die away. 



Fresh systems become formed by a continuation of the process of 

 budding, but the zooids of the secondary systems so formed are 

 sexual. The ova come to maturity before the spermatozoa, so that 

 cross fertilization takes place. 



In Botryllus we have clearly a rudimentary form of alternations 



1 It is not within tlie scope of this work to enter into details with reference to the 

 process of budding. The reader is referred on this head more especially to the papers 

 of Huxley (No. 16) and Kowalevsky (No. 22) on Pyrosoma, of Salensky (No. 35) on 

 Salpa, and Kowalevsky (No. 21) on Ascidians generally. It is a question of very great 

 interest how budding first arose, and then became so prevalent in these degenerate types 

 of Chordata. It is possible to suppose that budding may have commenced by the 

 division of embryos at an early stage of development, and have gradually been carried 

 onwards by the help of natural selection till late in life. There is perhaps little in the 

 form of budding of the Ascidians to support this view — the early budding of Didemnum 

 as described by Gegenbaur being the strongest evidence for it — but it fits in very well 

 with the division of the embryo in Lumbricus trapezoides described by Kleinenberg, 

 and with the not unfrequent occurrence of double monsters in Vertebrata which may 

 be regarded as a phenomenon of a similar nature (Eauber). The embryonic budding 

 of Pyrosoma, which might perhaps be viewed as supporting the hypothesis, appears to 

 me not really in favour of it; since the Cyathozooid of Pyrosoma is without doubt an 

 extremely modified form of zooid, which has obviously been specially developed in con- 

 nection with the pecuUar reproduction of the Pyrosomidffi. 



* The atrial spaces form somewhat doubtful exceptions to the rule. 



