vesicle soon divides into two, and its anterior part becomes 

 paired. The posterior vesicle also is converted into two, 

 so the number of brain vesicles increases to five. The 

 cavity of the anterior vesicle is the beginning of the 

 lateral ventricles of the brain, and its walls become the 

 hemispheres. Inside the second vesicle, the cavity of the 

 third ventricle appears. The third vesicle is the rudiment 

 of the four-hillocked mass. The fourth vesicle becomes the 

 cerebellum, and the fifth is the medulla oblongata. All 

 the brain vesicles which communicate among themselves at first 

 lie in one line, where the curving of the cephalic brain and 

 the reciprocal displacement of its division begins. The 

 subsequent development of the brain is described in the 

 first part (I 2m, 5aa, 9v, lOt, lip, 12g) . All sensory organs 

 are formed from the anterior part of the cephalic brain. 



Prior to other sensory organs, the eyes develop. They 

 are already detectable on the second day as two prominences 

 on the sides of the intermediate brain. Their connection 

 with the brain is narrowed, and at that time the eye rudi- 

 ments have the shape of vesicles which are situated on cored 

 stems; from the latter the optic nerves form, and from the 

 vesicles the eye balls. The vascular and hard eye membranes 

 develop with the splitting into layers of the initial single 

 cover of the eye, the same that takes place in the brain, 

 the cornea represents a part of the hard membrane; later 

 the anterior chamber of the eye develops under it (see v. I, 

 2n, 5bb, 6v, 7w, 9w, lOu, llq, 12h) . 



The ear is founded at the end of the second day; its 

 primary rudiment, according to Baer, is a protrusion of the 

 posterior part of the cephalic brain. 6 By what means the 

 auditory vesicle turns into a labyrinth remained unknown to 

 Baer; undoubtedly the auditory nerve forms like the optic 

 nerve. From the throat cavity to the ear, a protrusion 

 covered with mucous membrane grows ,. forming the eustachian 

 tube and the drum cavity. 



It is difficult to imagine how such a sharp-sighted and 

 careful observer as Baer could allow such a mistake. The 

 auditory vesicle (rudiment of the internal ear) is formed 

 from the increasing unlacing of the ectoderm. 



393 



