UROCHORDA. 35 



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 1 . The bud even- 

 tually 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 



degenerate types of Chordata. It is possible to suppose that budding may have com- 

 menced by the division of embryos at an early stage of development, and have gradu- 

 ally 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 Pyro- 

 somidEe. 



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



32 



