1271 
cause us no surprise if in an Annran a state of things were observed 
such as in Urodelans seems to be the rule, or the reverse, the 
difference between them not being fundamental, but only gradual. It 
would not be impossible that in one species at one time the first, 
prostomium 
Fig. 5. Larva of the sturgeon after Kuprrer from Herrwia’s Handbuch. 
1. limit of the gastrulation, 2. limit of the somatogenesis, 3. limit of the urogenosis. 
Beneath: Diagram of the interference of the gastrulation (a) with the action 
of the perianal (0) and the periporal (c) growing zones. 
at another the second case might be realized (comp. Dr Laner and 
IsHikawa on Megalobatrachus !). 
_I have spoken above of the candad movement of the neurenterie 
pore — blastopore stopping in front of the anus. In reality however 
there is no question of stopping. Although the anus seems to afford 
an insurmountable obstacle for the further backward growth of the 
stomodaeum = medullary tube, the activity of the periporal growing 
zone has not yet come to an end when the perianal growing zone has 
stopped working. There being no room however. within the soma 
for further extension, a protuberance of the body wall in front of 
the anus results, into which the stomodaeum = medullary tube 
grows out: the tail-knob (fig. 4c, text). Thus we see the tail of verte- 
brates originating by the periporal growing zone continuing its activity 
after the perianal has stopped. In this way the position of the anus 
in vertebrates is not terminal, as in Annelids, but at the root of 
the tail, which overgrows it and which owes its origin simply to 
the presence of the anus. Phylogenetically we have to imagine that 
the longitudinal growth of the stomodaeum (medullary tube) surpasses 
that of the soma, so that the cardiac (neurenteric) pore overtakes 
the anus and passes it. Just as in Annelids the position of the anus 
in Vertebrates is terminal in regard to the soma proper, the tail is 
an outgrowth of the dorsal side of the latter in a backward 
direction. According to this conception the ventral side of the tail 
- belongs to the dorsal side of the soma. In accordance with this the 
dorsal unpaired skinfold of the fish- and amphibia-larvae is continued 
