209 
supposing that the two cerebral ganglia, which originate 
separately, have not united by a commissure. They have 
remained separate, the oesophageal nerve-ring as a conse- 
uence not being closed anteriorly, and have passed, to- 
gether with the oesophageal connectives which form the com- 
munication with the ventral ganglion chain, into the optic 
vesicles with their stalks, while the anterior ganglia of the 
ventral chain give rise to the brain. He too, however, con- 
sidered the Vertebrate mouth as a secondary formation and 
found, with KUPFFER (1894), the palaeostoma again in the 
common invagination from which in Cyclostomes both the 
olfactory organ and the hypophysis take their origin. No 
facts, however, can be adduced in favour of any of these 
hypotheses and after their examination we can only con- 
clude that MINOT's statement that “il n'a été proposé 
aucune hypothèse ayant rapport à l'évolution de la tête 
du Vertébré aux dépens du type Annélide qui ne puisse 
encourir des objections insurmontables” has not yet lost 
its validity. 
All the above cited authors agree in that the Vertebrate 
mouth is a secondary one, but why the old mouth should 
have been lost and replaced by a new one, is not easily 
explained from their theories. GASKELL (1908, p 55), who 
derives the Vertebrates from Arthropodan ancestors in a 
very adventurous way, lets the cerebral and infra-oesophageal 
ganglia in the latter increase so much in size that the 
oesophageal ring is reduced to a very narrow passage for 
the gut, a process which finally resulted in a squeezing out 
the latter by the increasing nerve-mass (“antagonism between 
cephalization and alimentation”). The whole gut then passes 
into the medullary tube of Vertebrates (cf. above, p. 197). 
l hardly need point out once more how evident and natural an 
explanation of such a curious phenomenon as the loss of a 
mouth is afforded by my theory. But with reference also to 
the problem of the formation of the Vertebrate head the theory 
leads to most unexpected results, as 1 hope now to show. 
Praeoral lobe in Annelids.—In the development of the 
Annelids the umbrella or episphere of the trochophora- 
larva gives rise to the praeoral lobe or prostomium and to the 
cerebral ganglia of the worm, the subumbrella or hyposphere 
lengthens out into the segmented soma. It was KLEINENBERG 
(1886, p. 181) who first opposed these two regions of the 
Annelids body to each other and distinguished them as 
