522 



phases of their developmental history (i. e. so far as the minutest 

 details of structure, of which we are now dealing, are concerned), it 

 is almost as easy to study the mode of origin and the subsequent 

 history of the olfactory nerve in the human brain as in that of any 

 other vertebrate. 



To return to the cousideration of stage 30 in Lepidosiren — at 

 this time the brain begins to move away from the nasal sac, and some 

 of the cells, apparently derived from the latter, remain adherent to 

 the cerebral hemisphere (see Fig. 7, there are three flattened nuclei 

 labelled "Formatio bulbaris") to form the outer layer, or formatio 

 bulbaris, of the olfactory bulb — the Cappa olfactoria of some writers, 

 the olfactory ganglion of certain embryologists. 



At the same time the protoplasmic bridge uniting the nasal sac 

 to the cerebral hemisphere becomes stretched to form the olfactory 

 nerve (Fig. 7, Fila olfactoria). 



At this stage in Lepidosiren, as also in certain Elasmobrauchs 

 and Urodeles^), the olfactory bulb (i. e. the area of the hemisphere- 

 wall to which the formatio bulbaris is attached, and into which the 

 olfactory nerve is inserted) is a part of the lateral wall and not the 

 anterior extremity of the hemisphere. 



But very soon the relations of the bulb and the form of the whole 

 hemisphere undergo very curious modifications, which have been de- 

 scribed in Lepidosiren by Graham Kerr 2) and in Ceratodus by 

 Sewertzoff ^). 



In the course of development the olfactory bulb (which still forms 

 a constituent part of the wall of the lateral ventricle and is not drawn 

 out or evaginated to form a projection containing a distinct olfactory 

 ventricle) moves upwards to occupy a position, which is morphologi- 

 cally the cephalic pole of the cerebral hemisphere. That it does not 

 actually form the anterior extremity is due to the fact that the region 

 which in most vertebrates forms a small area of the ventral surface 

 of the hemisphere in the interval between the olfactory bulb (or peduncle) 

 and the optic chiasma — the tuberculum olfactorium in fact — undergoes 

 an enormous and most precocious development, so as to form a large 

 sac, bulging forward below the olfactory bulb and nerve and becoming 



1) Compare v. Kupffer's representation of the condition found in 

 the larval Necturus (op. cit. supra, Fig. 190, p. 177). 



2) Op. cit. supra, Quart. Journ. Micr. Sc, see especially Pis. 26 

 and 27. 



3) Zur Entwickelungsgeschichte des Ceratodus Forsteri. Anatom. 

 Anzeiger, Bd. 21, 1902 — see especially p. 605 and Fig. 4. 



i 



