DEVELOPMENT. 



199 



known as the epencephalon, and the after-brain, or metenceph- 

 alon. 



What parts of the brain do these five vesicles respectively orig- 

 inate ? 



I. Anterior 

 primary 



vesicle. 



II. Middle 

 primary 

 vesicle. 



1. Prosencephalon. 



2. Thalamencephalon. 



3. Mesencephalon. 



III. Posterior 4. Epencephalon. 



primary 

 vesicle. 



5. Metencephalon. 



C Cerebral hemispheres, cor- 

 j pora striata, corpus callo- 

 1 sum, fornix, lateral ven- 

 (^ tricles, olfactory bulb. 

 ( Thalami optici, third ven- 

 | tricle, optic nerve. 

 C Corpora quadrigemina, 



crura cerebri, aqueduct 

 (^ of Sylvius, 

 f Cerebellum, pons Varolii, 



anterior part of fourth 

 ( ventricle. 



| Medulla oblongata. fourth 

 | ventricle, auditory nerve. 



What is the primary optic vesicle ? 



About as soon as the cerebral vesicles are distinctly formed a 

 budding of two projections one from either side of the anterior 

 vesicle occurs. These are the primary optic vesicles. They are 

 formed before the vesicles which make the hemispheres (prosen- 

 cephalon). The projections approach the external epiblast, and at 

 that period consist of a finger-like process having a globular dilata- 

 tion at the end. This subsequently forms the optic nerve and the 

 retina. 



How is the crystalline lens formed? 



Opposite the optic vesicle the superficial epiblast is depressed 

 and forms a sort of pit, forcing the optic vesicle to fold in upon 

 itself. The follicle of epiblast is shut off at the surface, and a ball 

 of its substance left in the cup of the infolded optic vesicle. This 

 ball forms the rudimentary lens, and the anterior layer of the ves- 

 icle is the retina. 



How are the other tissues of the eye evolved ? 



The muscular and vascular structures, as well as the connective 

 tissue and humors, are derived from the mesoblast. which in part 

 enfolds the ocular vesicle and in part enters it between the lens 



