300 THE GERM-LAYERS AND EVOLUTION 



to the exterior by pores. This longitudinal canal was 

 lost in Invertebrates, but persisted in Vertebrates as the 

 pronephric duct, while the pores remained in Invertebrates 

 and disappeared in Vertebrates ; (4) that the tracheae of 

 Arthropods, as well as the canal of the central nervous system 

 in Vertebrates, are to be traced back to certain ectodermal 

 pits in the diploblastic ancestor comparable to the sub-genital 

 pits of the Scyphomedusae. These ectodermal pits were all 

 originally respiratory organs. " The essence of all these 

 propositions," he writes, " lies in the fact that the segmented 

 animals are traced back not to a triploblastic unsegmented 

 ancestor, but to a two-layered Ccelenterate-like animal with 

 a pouched gut, the pouching having arisen as a result of the 

 necessity for an increase in the extent of the vegetative 

 surfaces in a rapidly enlarging animal (for circulation and 

 respiration)" (p. 47). " I have attempted to show," he 

 writes further on, " that the majority of the Triploblastica . . . 

 are built upon a common plan, and that that plan is revealed 

 by a careful examination of the anatomy of Coelenterata ; 

 that all the most important organ-systems of these Triplo- 

 blastica are found in a rudimentary condition in the Coelen- 

 terata ; and that all the Triploblastica referred to must be 

 traced back to a diploblastic ancestor common to them 

 and the Ccelentcrata " (p. 68). The main assumption was 

 that the neural or blastoporal surface must be homologous 

 throughout the Metazoa, though it was dorsal in the Chordata, 

 ventral in the Annelida and Arthropoda. He derived the 

 central nervous system of the Chordata from the circumoral 

 ring of the common ancestor by means of the hypothesis 

 that both the pre-blastoporal and the post-blastoporal parts 

 of it disappeared. 1 



The characteristic relation of the central nervous system 

 to the blastopore in Annelida and Vertebrates had already 

 been pointed out by Kowalevsky, 2 who had also sketched 

 a theory of the common descent of these two phyla from 

 an ancestral form in which the nervous system encircled 

 the blastopore. 



1 See further the same author's article "Embryology" in the Ency. 



/.Vv7., vol. xi., Mil) r<L ( '.iinKrid-v, \')lo. 



- Arch.f. mikt: An>if., xiii., pp. 181-204, 1877. 



