. Anatomischer Anzeiger. Anatomy, Comparative; Anatomy, Comparative. 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
. Anatomischer Anzeiger. Anatomy, Comparative; Anatomy, Comparative. 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 on
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