402 GENERATION. 



canal, the aqueduct of Sylvius, which is left in the develop- 

 ment of the middle vesicle. At about the fourth month, 

 there is a deposition of nervous matter in front and above, 

 forming the pons Varolii. 1 



In Fig. 42 (O y o\ it is seen that the vesicles for the organs 

 of vision appear very early, as lateral offshoots of the anterior 

 cerebral vesicle. These gradually increase in size and ad- 

 vance anteriorly, as development of the other parts progresses. 

 We shall see, when we come to study the development of the 

 face, that the eyes are situated at first at the sides of the 

 head, gradually approaching the anterior portion. At the 

 extremity of each of these lateral prolongations, a rounded 

 mass appears, which becomes the globe of the eye. The su- 

 perficial portions of the globe are developed into the scle- 

 rotic and the cornea, which seem to be formed of a process 

 from the dura mater. The pedicle attached to the globe be- 

 comes the optic nerve. The iris is developed at about the 

 seventh week, and is at first a simple membrane, without 

 any central opening. As the pupil appears, it is closed by a 

 vascular membrane, which probably belongs to the capsule of 

 the crystalline lens, and is called the pupillary membrane. 

 This membrane gradually disappears by an atrophy extending 

 from the centre to the periphery. It attains its maximum of 

 development at the sixth month, and disappears at the seventh 

 month. The vitreous humor is formed of the fluid contents 

 of the optic vesicle. The crystalline lens is regarded as a 

 product of the tegumentary layer. At the tenth week, we 

 observe the beginning of the formation of the eyelids. These 

 meet at about the fourth month and adhere together by their 

 edges. 2 In many mammals, the eyelids remain closed for a 

 few days after birth ; but they become separated in the hu- 

 man subject in the later periods of foetal life. 



It is probable that the vesicle which becomes developed 

 into the internal ear is formed independently ; at ]east, cases 



1 LONGET, Traite de physiologic, Paris, 1869, tome iii., p. 893 

 8 Ibid., p. 896. 



