Embryology. 137 



published only a few (eleven) papers, but some of these 

 were of great value, especially his account of the 

 development of Hydra (1872), of Lumbricus trapezoides 

 (1878), and of the Polychaete worm Lopadorhynchus 

 (1886). In his memoir on Lopadorhynchus he dealt 

 very severely with the conception of the mesoderm as 

 an independent germinal layer, and sketched his theory 

 of the substitution of organs. This may be explained 

 by taking a concrete instance. 



In all Vertebrate embryos there is, for some time at 

 least, a supporting axial rod or notochord, developed 

 along the dorsal median line of the primitive gut. This 

 persists throughout life in the lancelet and lamprey and 

 a few old-fashioned types, but from Fishes onwards it 

 is gradually replaced in development by the backbone. 

 The notochord does not become the backbone, which has 

 a different (so-called mesodermic) origin, but is replaced 

 by it. The notochord is a temporary structure, around 

 which the vertebral column is constructed, as a tall brick 

 chimney might be built around an internal scaffolding of 

 wood. Now, what is the relation between the more 

 primitive axis or notochord and its more effective sub- 

 stitute the backbone, seeing that the former does not 

 become the latter? Kleinenberg's suggestion was that 

 the notochord supplies the stimulus, the necessary de- 

 velopmental condition, for the formation of the backbone 

 when suitable materials are forthcoming. Of course 

 we require to know more about the way in which the 

 old-fashioned structure prepares the way for and stimu- 

 lates the growth of its future substitute, but the general 

 idea of one organ leading on to another is suggestive. 

 It is consistent with our general conception of develop- 

 ment that each stage supplies the necessary condition 

 for the next ; it helps us to understand more clearly how 

 new structures, too incipient to be functional, and old 

 structures, too transitory to be of direct use, may per- 

 sist ; in short, it makes the process both of development 

 and evolution more intelligible. 



Kleinenberg maintained that the Annelids possessed 

 two quite distinct nervous systems, one for the larva, 

 and the other for the adult, which are not homologous 



