DEVELOPMENT OF ORGANS OF SENSE. 603 



arteriosus and ductus venosus, as well as the umbilical vessels ; 

 so that the two streams of blood which arrive at the right 

 auricle by the superior and inferior vena cava respectively, 

 thenceforth mingle in this cavity of the heart, and passing 

 into the right ventricle, go byway of the pulmonary artery to 

 the lungs, and through these, after purification, to the left 

 auricle and ventricle, to be distributed over the body. (See 

 chapter on Circulation.) 



Development of the Nervous System. 



The mode in which the rudimentary structures of the cere- 

 bro-spinal nervous system are formed, has been already stated 

 (p. 582). The dorsal laminae, the inner borders of which close 

 in and form the canal of the spinal cord, seem to leave a fis- 

 sure in the situation of the medulla oblongata. Between this 

 and the most anterior extremity of the canal, three vesicular 

 enlargements, the vesicles of the brain, are developed (see Fig. 

 217), and from these again are developed the following parts : 



From the anterior primary vesicle the optic thalami, cor- 

 pora striata, the third ventricle, and the cerebral hemispheres, 

 together with some other parts in connection with those above 

 named, as the corpus callosum, fornix, &c. 



From the middle primary vesicle the corpora quadrigem- 

 ina and crura cerebri, with the aqueduct of Sylvius. 



From the posterior primary vesicle the cerebellum, pons 

 Varolii, medulla oblongata, &c. 



Development of the Organs of Sense. 



The eye is in part developed as a protruded portion of the 

 first primary cerebral vesicle ; while passing backwards, and 



Diagram of development of the lens. ABC. Different stages of development. 

 1. Epidermic layer. 2. Thickening of this layer. 3. Crystalline depression. 4. 

 Primitive ocular vesicle, its anterior part pushed back by the crystalline depression. 

 5. Posterior part of the primitive ocular vesicle, forming the external layer of the 

 secondary ocular vesicle. 6. Point of separation between the lens and the epidermic 

 layer. 7. Cavity of the secondary ocular vesicle, occupied by the vitreous. 



