THE SENSE ORGANS 121 



first dii'ected baokwards ; in the later stages it grows forwards 

 and forms a rounded vesicle connected with the brain by a long 

 pigmented stalk ; when the skull develops it cuts off the vesicle 

 from the stalk, the former remaining as a small rounded body 

 outside the skull, while the stalk persists as a slender pigmented 

 tract within the cranial cavity. 



In front of the pineal body, and at the anterior end of the 

 fore-brain, the roof is thrown iato folds which hang down into 

 the ventricle forming a choroid plexus, CP', similar to that in 

 the medulla. 



The anterior end of the fore-brain grows forwards as a median 

 thin- walled cerebral vesicle, from which at a slightly later stage 

 the cerebral hemispheres, CH, arise as a pair of hollow out- 

 growths ; the foramina of Monro being the apertures of com- 

 munication between the lateral ventricles or cavities of the 

 hemispheres, and the third ventricle. The anterior ends of the 

 hemispheres grow forwards as the olfactory lobes, which become 

 fused together in the median plane. 



The peripheral nervous system. The cranial nerves and the 

 dorsal roots of the spinal nerves are formed from the deeper or 

 nervous layer of the epidermis. They appear to arise as lateral 

 outgrowths from the edges of the neural plate, and may be 

 recognised at a very early stage, while the neural groove is still 

 shallow and open ;■ they are, therefore, at their first appearance 

 continuous with the brain or spinal cord. 



The ventral roots of the spinal nerves arise later than the 

 dorsal ones, as outgrowths from the cord near its ventral sur- 

 face. They are at first independent of the dorsal roots, but 

 soon become connected with these. 



G. Development of the Sense Organs. 



The organs of special sensation are developed from the deeper 

 or nervous layer of the epiblast, and become connected with 

 their respective nerves at a very early stage of their formation. 



The derivation of the sense organs from the epiblast is 

 explained by the fact that they are concerned with the appre- 

 ciation of the presence and nature of external objects, and are 

 therefore necessarily formed on the surface of the body. They 

 may be regarded as specially modified portions of the epidermis. 



The Nose. The olfactory organs appear at a very early stage 

 as paired thickenings of the nervous layer of the epiblast at 



