622 



THE KEKVOUS SYSTEM. 



the roof. Into this secondarily formed caudal extension of the chorioid lamina the 

 invagination that commenced in the roof of the foramen interventriculare extends 

 untif it reaches the inferior extremity of the deep cleft separating the cerebral 

 hemisphere from the thalamus (Fig. 548, A). Below this point the thalamus 

 remains in uninterrupted continuity with the floor of the cerebral hemisphere (Cs), 

 which is becoming thickened to form the corpus striatum. 



At a very early stage in the development of the embryo, long before there is any 

 sign of the hemisphere vesicles, the ectoderm upon each side of the anterior neuro- 



pore (see p. 500) becomes 



( Fissura chorioidea. thickened to form the area 



olfactoria (see Fig. 440, D, p. 

 501). Certain of the epithelial 

 cells in this area become con- 

 verted into bipolar sensory cells, 

 which become specially adapted 

 to be affected byicertain kinds of 

 air-borne chemical stimuli that 

 awaken a consciousness of smell. 

 These cells always remain in situ 

 in the olfactory epithelium, just 

 as the most primitive sensory 

 cells do in Hydra (Fig. 439, p. 



497). But other nerve -cells 

 FIG. 549. DIAGRAM OF A TRANSVERSE SECTION THROUGH A , , . , 



FCETAL BRAIN TO SHOW THE INVAGINATION OF THE ROOF seem to be derived trom the 

 THROUGH EACH iNTERVENTRicoLAR FORAMEN. area olfactoria which do not 



remain in the parent epithelium, 



but become attached to the adjoining part of the neural tube. These cells form 

 the olfactory ganglion, which acts as the receptive organ for the impressions brought 

 into it by the processes of the sensory cells in the olfactory epithelium; and the 



--Pallium. 



.-i-\ -Venfriculus lareralis. 

 _ Lamina chorioidea 

 venhriculi la^ralis. 



-Foramen inrervenrriculare. 



Lamina chorioidea 

 venrriculi rerrii. 

 Corpus srriafum. 



-Third ventricle. 



Floor plafe. 



HYPOPHYSEAL DIVERTICULUM 

 RECESS CHIASMA 



FIG. 550. Two DRAWINGS BY His, ILLUSTRATING THE DEVELOPMENT OF THE HUMAN BRAIN. 



A, Median section through a foetal human brain in the third month of development. 



B, Schema showing the directions in which the cerebral hemisphere expands during its growth 



P.M.H. Pars mamillaris hypothalami. M. Mamillary region. 0. Occipital lobe. 



P.O.H. Pars optica hypothalami. F. Frontal lobe. T. Temporal lobe. 



P. Parietal lobe. 



area of the neural tube to which it bscomes attached is destined to become part of 

 the cerebral hemisphere. At the end of the first month this portion of the hemi- 

 sphere becomes drawn out as a hollow protrusion, the distal end of which is 

 coated with a layer of olfactory ganglion and is known as the bulbus olfactorius ; 

 the rest forms a peduncle. In the course of its subsequent development in the 

 human brain (though not in those of most mammals) the cavity in the bulb and 

 peduncle becomes completely obliterated. The peduncle becomes so greatly 

 elongated and attenuated that, to the unaided eye, it appears to be wholly formed 



