GENERATION AND DEVELOPMENT 



291 



corpus callosum, are developed by the growth of fibres across the middle 

 line. 



The Hippocampus major is formed by the folding in of the grey mat- 

 ter from the exterior into the latter ventricles. The essential points in 



FIG. 448. Diagrammatic horizontal section of a Vertebrate brain. The figures serve both for 

 this and the next diagram. Mb, mid-brain: what lies in front of this is the fore, and what lies be- 

 hind, the hind brain; Lt, lamina terminals; Olf, olfactory lobes; Hmp, hemispheres; TH.E, thalam- 

 encephalon; Pn,, pineal gland; Py, pituitary body; FM, foramen of Munro; cs, corpus striatum; 

 Th, optic thalamus; CO, crura cerebri: the mass lying above the canal represents the corpora quad- 

 rigemina; C6, cerebellum; I IX, the nine pairs of cranial nerves; 1, olfactory ventricle; 2, lateral 

 ventricle ; 3, third ventricle ; 4, fourth ventricle ; + , iter a tertio ad quartum ventriculum. (Huxley.) 



'the structure and arrangement of the various parts of the brain, are dia- 

 grammatically shown in the two accompanying figures (Figs. 448, 449). 



DEVELOPMENT or THE ORGANS OF SENSE. 



Eye. Soon after the first three cerebral vesicles have become distinct 

 from each other, the anterior one sends out a lateral vesicle from each 

 side (primary optic vesicle), which grows out toward the free surface, its 

 cavity of course communicating with that of the cerebral vesicle through 

 the canal in its pedicle. It is soon met and invaginated by an in-grow- 

 ing process from the epiblast (Fig. 450), very much as the growing 

 tooth is met by the process of epithelium which produces the enamel 

 organ. T^his process of the epiblast is at first a depression which ulti- 

 mately bcomes closed in at the edges so as to produce a hollow ball, which 

 is thus completely severed from the epithelium with which it was origi- 

 nally continuous. From this hollow ball the crystalline lens is developed. 



