162 



Fig. 7. Diagrams of sagittal sections of stages in 

 the development of a craniate cliordate shewing the 

 growth centres as described hi the text. A Gastraa 

 B Balanoglossus stage. C Tailed chordate stage. 



In those which never possess a blastopore the same dying out of 

 the activity of a ventral part of the deuterogenetic area occurs and 

 in this area the anus arises as a later formation. 



The dorsal part continues its activity for some time, giving rise 

 to the post-anal part of the vertebrate, the tail. 



So I read the main 

 A^ B c facts of vertebrate em- 



bryology. 



How are they to 

 be interpreted from the 

 evolutionist point of 

 view? 



All that this record 

 tells us is that there 

 was a multicellular or- 

 ganism — with radial 

 symmetry, in fact a coe- 

 lenterate or Haeckel's 

 gastrsea. Whether this 

 arose by invagination 

 orby other means does 

 not matter ; I personally 

 would follow Lankester and Hubrecht. It had a digestive cavity 

 open to the exterior at one point (the blastopore). 



Elongation of this organism in the Vertebrate phylum came about 

 not so much by general growth but by the more active growth round 

 the lips of the blastopore — possibly not in the adult stage, but as 

 an embryonic modification, due perhaps even at that time to the 

 greater purity of the protoplasm in that region — or to its more 

 favourable position. 



This produced the cylindrical creature. . 



By now no doubt the general vertebrate characters were fore- 

 casted — concentration of the nervous tissue over the future dorsal 

 area, development of coelom and formation of the notochord. 



The mouth, as development shews in every case in the chordate 

 phyla, is a new opening. The direct embryological evidence in favour 

 of the mouth being derived from the anterior end of the blastopore is 

 entirely wanting. Sedgwick who in 1884 propounded the Actinia 

 mouth theory now adopted by Hubrecht practically admitted in his 

 paper of 1892 that the evidence is only indirect. On the other hand, 

 embryological evidence of its origin as a new perforation in the verte- 

 brates like the perforation of the gill slits is overwhelming. 



