116 



NEKVOUS SYSTEM 



mesencephalo n 



cerebellum 



The hemispheres in an embryo of the fourth week (fig. 148) still form a single 

 rounded swelling, but the position of the future separation is marked by a longi- 

 tudinal ridge, which, however, does not reach the lower and anterior portion. Here 

 an area on each hemisphere is clearly marked off by a slight lateral furrow as the 

 rudiment of the rhinencephalon. When the hemisphere is looked at from within, 

 it will be observed that two areas can be distinguished an upper rounded, the 

 rudiment of the pallium, 1 and a lower triangular, the rudiment of the corpus strmtum. 

 On the latter are seen three ridges running on to the mesial aspect of the rhin- 

 encephalic area. The mouth of the vesicle (the future foramen of Monro) has the 

 following lips, which it is important to distinguish : (1) pallio-thalamic, where 

 pallial and thalamic walls join one another ; (2) strio-thalamic, where corpus striatum 

 and thalamus are continuous ; (3) strio-hypothalamic, where corpus striatum 



and hypothalamus meet ; and 



c. hemisphere opened (4) lamina tcrminalis, where rhin- 



encephalon is continuous with 

 rhinencephalon, and pallium with 

 pallium. 



In the early part of the fifth 

 week the hemispheres begin to 

 expand (figs. 153, 154). Their ex- 

 pansion takes place, like that of 

 a growing soap-bubble, in every 

 direction from the stationary mouth 

 (foramen of Monro). The lamina 

 terminalis thus comes to lie at the 

 bottom of a fissure between the two 

 hemispheres which is occupied by 

 a lamella of mesenchyme, the 

 common rudiment of the mesial 

 pia and the falx cerebri ; the 

 pallio-thalamic border is at the 

 same time converted into an 

 angular arched fold which lies at 



the bottom of a cleft between the 



The right hemisphere has been opened up to show Posterior part of the expanding 



the cavity of the vesicle and the inner aspect of its vesicle and the thalaniUS. 



mesial wall. The inward projecting fold round the Th rhinencephalon is now 



stalk is the hippocampus; the angular fold m the . 



posterior lobe is the calcar avis. marked ore very distinctly from 



the pallium by a lateral furrow 



(external rninal fissure}. The pallial vesicle soon becomes bean-shaped (fig. 155), 

 the hilum being represented by a depression above the rhinencephalon. This 

 depression becomes the fossa of Sylvius, and corresponds to the stalk of the 

 hemisphere i.e. the point of union of corpus striatum and thalamus. The 

 hemisphere -vesicle, anchored as it were to the two ends of the rhinencephalon, 

 and expanding in all directions, becomes folded round the stalk. Its cavity is 

 consequently horseshoe-shaped, and the horns of the semilune represent the 

 future frontal and temporal horns of the lateral ventricle. There is no posterior 

 horn as yet. It is developed later, when the vesicle has still further expanded 

 backwards to form the occipital lobe. 



1 The terms rhinencephalon &n& pallium are used in a limited and ontogenetic sense. The division 

 is in a measure arbitrary, as we shall see that a part of the pallium in this sense (limbic lobe) become* 

 closely related to the rhinencephalon, forming with it the 'rhinencephalon' of Elliot-Smith's definition. 

 Ontogenetically, however, the above subdivision of the hemisphere-rudiment into rhinencephalon, 

 pallium, and corpus striatum is not only justified by fact, but necessary in description. 



