208 THE ANCESTRY OF VERTEBRATES 



between the yolk-cell-mass and the extremity of the tail. 

 In fishes this case is fairly generally met with. As an 

 example may be mentioned the sturgeon (fig. 43). Many 

 Teleosteans might also be mentioned here, in whose larvae 

 the place of the anus varies considerably, which is of 

 importance in determining the species. 



Let us now turn to the periporal growing zone which 

 causes the growing out of the stomodaeum, the medullary 

 tube or the medullary plate, together with the backward 

 movement of the cardiac pore (Annelids), the neurenteric 

 pore (Chordates) or the blastopore. Organs or processes 

 that are of much importance for the structure of the adult 

 animal often appear precociously in ontogeny. In Lamel- 

 libranchia, e.g., the shell-gland invaginates during gastrula- 

 tion, though the gastrulation-process is no doubt phyloge- 

 netically much older Thus also the activity of the periporal 

 growing zone and the backward movement of the cardiac 

 pore associated with it begin very precociously, viz: during 

 gastrulation, when the future cardiac or neurenteric pore 

 is still the blastopore. The interference of the contraction 

 of the blastoporic rim with the backward movement of the 

 blastopoie causes the caudad eccentric closure of the blasto- 

 pore which is so typical of Chordates. The action of the 

 periporal growing zone, as long as the tube-formation has 

 not set in, results not in the production of a stomodaeal 

 or medullary tube, as is afterwards the case during the 

 urogenesis, but provisionally in the formation of the medul- 

 lary plate. As I have expressed before (cf. p 191), the growing 

 out of the stomodaeum into the medullary tube is thus in its 

 first, and somatogenetic, phase to be imagined as projected on 

 a plane, the dorsal plane of the embryo When the blastopore 

 has narrowed to a slit and the tube-formation begins, in the 

 form of the appearance of the medullary folds, the caudad 

 movement of this slit-like blastopore, as stated above, conti- 

 nues nevertheless, probably with undiminished speed, though 

 only over a short distance — as indeed might be expected 

 from the short duration of this phase. Further than the anus, 

 however, this backward movement can not go; phylogene- 

 tically: the stomodaeum of the Annelid, growing outback- 

 wards, finally reaches the anus. If now the movement stops 

 slightly in front of the anus, there will be no relation whatever 

 between neurenteric pore (blastopore) and anus (fig. 44 a), as 

 we stated was the case with the frog. If the movement continues 



