161 



If Professor Hubrecht will allow me I will duriag the rest of this 

 article use these words, but not as substitutes for Kephalogenesis and 

 Notogeuesis, because they do not express quite the same ideas though 

 called forth by the recognition of the same phenomena. 



Moreover, Hubrecht's names I do not like, because although the 

 greater part of the head is included in the kephalogenesis, not all the 

 head is so included; and also more than the head is included in proto- 

 genesis e. g. a considerable part of the gut and the heart, while deutero- 

 genesis includes not only the back, but also, for a time, the sides and 

 ventral walls and even a small part of the gut. 



To turn now to the more general aspect. As I read the develop- 

 mental history of the Vertebrates, the course is as follows: 



An organism is produced as the result of a single centre of cell 

 production (the segmentation of the egg), which exhibits strong ten- 

 dencies to a radial symmetry, as seen in the blastula of Amphioxus, 

 blast, of Amphibia, blast, of Dipnoi etc., in the morula of Mammals, 

 and the early segmentation stage in meroblastic eggs. 



This, still as the result of the primary centre of activity (proto- 

 genesis) becomes a two-layered organism with a cavity, which is pro- 

 duced by invagination (Amphioxus) or by separation of cells by various 

 means (e. g. a form of splitting in some Amphibia or by accumulation 

 of fluid among the cells [Amuiota]). To this cavity and this cavity 

 alone I would restrict the term Archenteron (cf. Assheton, 2, p. 224). 



After this cavity has completely formed (Amniota, Acrania?) or 

 after it has begun but not yet completely formed (Amphibia, Pisces), 

 the secondary area of cell proliferation (deuterogenesis) arises. In 

 some embryos there is an opening from the archenteron to the ex- 

 terior, the blastopore, but not in all. In those which have a blasto- 

 pore, the deuterogenetic centre arises round this aperture forming its 

 lips, and adds on new material to the radially symmetrical animal at 

 all points of contact. This tends to produce a cylindrical organism 

 (later a bilateral symmetry is acquired) with a terminal aperture in 

 some cases (Amphioxus, Amphibia, etc.); but after a short time the 

 activity of the deuterogenetic centre dies out in those which have a 

 blastopore except at its extreme dorsal edge, where, however, it con- 

 tinues active and proliferates new material to those parts only of the 

 now much elongated embryo which form the dorsal part, while the 

 ventral part in which the blastopore lies is, so to speak, left behind 

 — no additional segments being added on here — and the blasto- 

 pore becomes the anus. (Cf. Assheton and Robinson, 23, and Sedg- 

 wick, 25.) 



Anat. Am. XX VII. Aufsätze. 11 



