254 



PRINCIPLES OF EMBRYOLOGY 



two, SO that the brain comes to consist of five vesicles, whose names are 

 shown in Fig. 12.2. 



The division of the forebrain into two starts early, but progresses slowly. 

 Even before it has properly started, the formation of the eyes begins. 

 They are developed from the region which will eventually form part of 

 the second forebrain vesicle (the diencephalon), and are originally merely 

 exaggerated lateral swellings of the brain tube. These bulge out through 



FOREBRAIN 



Tehnciphalon 



Diencephalon 



MIDBRAIN — lAesenctphalon 



HINDBRAIN 



Metencepkalo^ 

 Muencephalon 



Masai 

 placode 



Lens 

 placode 



acodt 



Figure 12.2 



Diagram of the various regions of the brain, the eye-cups, and the nasal, lens 

 and ear placodes. 



the mesenchyme which lies between the brain and the ectoderm (Fig. i.i, 

 p. 7), and come up against the latter. At this stage, two things 

 happen. The first is the formation of the 'optic cup' ; the swelling be- 

 gins to be transformed into a roughly mushroom-shaped structure, with 

 a rather narrow stalk leading up to the brain while at the other end the 

 part wliich comes into contact with the ectoderm is folded inwards rather 

 as one pushes in the foot of a sock before puttiag it on. The cavity thus 

 formed is the eye-cup, and is approximately circular except for a groove, 

 the choroid fissure, on the ventral side. The layer of tissue wliich lines the 

 cavity of the cup becomes the light-sensitive retina, and the outer layer or 



