ORIGIN AND STRUCTURE OF THE HEAD 43 



with the optic organs, and in supplying with nerves only 

 organs of sense. Its connection with the olfactory organs is 

 an argument in the same 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 

 form s." "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 structur e." A little further BALFOUR 

 remarks: "there is strong embryological evidence that the 

 mid- and hind-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 in 

 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 



