120 ARTHUR DENDY. 



Stage R. 



Of Stage R^ however, I have numerous examples^ as the 

 embryo passes a long period in this condition. According to 

 my classification of the embryos^ this is the last stage but one 

 before hatching. The embryo is very advanced. The limbs 

 and digits are well formed, and pigment is present in the 

 integument, forming a characteristic embryonic pattern of 

 transverse bands and longitudinal stripes. 



The upper surface of the head of an embryo at about the 

 commencement of Stage R is represented in fig. 13. When 

 this figure is compared with fig. 12 it will be seen that owing 

 to the straightening out of the cerebral flexure the optic lobes 

 and cerebral hemispheres have become closely approximated, 

 and the length of the thalamencephalon thereby greatly 

 reduced, its roof appearing as a dark-looking, diamond-shaped 

 area between the two.^ In the centre of this area the parietal 

 eye appears as a round white spot with an intensely black 

 border, the white centre representing the lens, and the black 

 border the now deeply pigmented margin of the retina. As 

 far as one can judge, the parietal eye has now become median 

 in position. 



Topography of the Brain in the Neighbourhood of 

 the Third Ventricle. — As the brain has progressed greatly 

 in development since the last stage described, it will be 

 desirable to briefly describe the topogra])hical relations of the 

 parts in the neighbourhood of the third ventricle before 

 proceeding any further, and this may best be done with the 

 aid of a series of transverse sections. 



Fig. 32 represents a transverse section taken a short way in 

 front of the foramina of Monro. It shows the anterior 

 commissure {Com. Ant.) passing across above the anterior end 

 of the third ventricle. Above the anterior commissure is a 

 crescentic band of fibres {Com. Man.) uniting the two inner 



1 This dark patch, however, does not exactly mark the limits of the roof of 

 the thalamencephalon, for it will be shown later on that the parietal eje lies 

 over the hinder part of the roof rather than over the centre as it appears 

 externally. 



