229 
with the optic organs, and in supplying with nerves only 
organs of sense. Its connection with the olfactory organs is 
an argument in the sante direction.” 
“The evidence at our disposal appears to me to indi- 
cate that the third nerve belongs to the cranio-spinal series 
of segmental nerves, while the optic and olfactory nerves 
appear to me equally clearly not to belong to this series. 
The mid-brain, as giving origin to the third nerve, would 
appear not to have been part of the ganglion of the prae- 
oral lobe.” 
“These considerations indicate with fair 
probability that the part of the head con- 
taining the fore-brain is the equivalent 
of the praeoral lobe of many Invertebrate 
forms” “It must however be admitted that this part of 
the head is not sharply separated in development from that 
behind; and though the fore-brain is usually differentiated 
very early as a distinct lobe of the primitive nervous tube, 
yet that such a differentiation is hardly more marked than in 
the other parts of the brain. The termination of the notochord 
immediately behind the fore-brain is, however, an argument 
in favour of the morphological distinctness 
of the latter structure.” A little further BALFOUR 
remarks: “there is strong embryological evidence that the 
mid- and hínd-brains had primitively the same structure 
as the spinal cord.” All this is in the most complete accord- 
ance with the results to which my theory !eads. 
Olfactory organ. — A third sense-organ in Craniates which, 
in consequence of my theory, appears to be derivable from 
the corresponding organ in Annelids is the olfactory organ. 
In its simplest and most primitive form we find it i 
Craniates as a pair of ectodermal invaginations in front of 
the mouth internally coated with cilia. 
Some hypotheses regarding the phylogeny of the olfactory 
grooves may be mentioned here. DOHRN (1875) considered 
them as originally being a pair of praeoral gill-clefts, an 
idea which afterwards was worked out especially by MILNES 
MARSHALL (1879), but rejected by BALFOUR (1881) and 
GEGENBAUR (1887 p. 9, 20) who emphasize that the olfactory 
grooves are wholly ectodermal while the gill-pouches are 
produced by the entoderm. BEARD (1885) views in the 
olfactory grooves the branchial sense-organs of a pair of 
“non-existing” praeoral gill-slits which, however, remain 
