514 



THE HUMAN EMBRYO. 



of the hinder part of the medulla oblongata being in contact 

 with each other ; while, on the ventral surface of the brain, the 

 angle of the flexure, which marks the place at which the pons 

 Varolii will appear, almost touches the infundibulum. 



The cerebral hemispheres have increased considerably, and 

 now overlap nearly half the sides of the thalamencephalon. 

 Each hemisphere is somewhat reniform in outline, the notch or 

 hilum, opposite the optic stalk, being the commencement of the 

 Sylvian fissure. 



The third month. By the end of the third month (Figs. 218- 

 221). the cerebral hemispheres are by far the largest portions of 

 the brain, and completely cover the thala- 

 mencephalon. The Sylvian fissure forms a 

 conspicuous notch in the ventral border of each 

 hemisphere ; and the sulci are commencing to 

 appear as grooves on the surface. The mid- 

 brain is still small and undivided; but the 

 cerebellum has increased very considerably in 

 size. 



The fourth month (Fig. 222) is chiefly 

 marked by a still further increase in size of 

 the cerebral hemispheres, which now completely 

 cover the thalamencephalon, and overlap part 

 of the mid-brain as well. The cerebellum has 

 increased considerably in size, and the trans- 

 verse fibres of the pons Varolii are commencing 



to form. 

 FIG. 218. A Hu- 

 man Foetus three 

 months old, dis- 



sected from the ,, /-,-,. n oo\ , i i i i i 



dorsal surface to month (rig. 223) the cerebral hemispheres over- 



expose the brain ] a p the cerebellum and project some distance 

 and spinal cord. , a '*. m. a i a j j 



(From Kolliker.) beyond it. The Sylvian fissure is a deep and 



Natural size. conspicuous depression on the outer surface 

 re! of each hemisphere ; the mid-brain is divided 

 into the corpora quadrigemina by two fissures, 

 longitudinal and transverse respectively ; and the optic chiasma, 

 pons Varolii, olivary bodies, and other parts of the adult brain 

 are well established. 



The sixth month. By the end of the sixth 





