[13] It is not within the 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 (Rauber). 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 connection with the peculiar reproduction of the Pyrosomidæ. 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 corresponding layers to those in the embryo[14]. 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. [14] The atrial spaces form somewhat doubtful exceptions to the rule. 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.