600 DEVELOPMENT [CHAP. XLH. 



lies in front of this, and at first is extremely small ; it bears a 

 proportion to the rest of the encephaloii not greater than that 

 which the small, unimportant cerebral lobes of the adult fish bear 

 to its entire cerebrum. The prosencephalon soon, however, 

 increases in size, and becomes much larger than all the others. 



Our friend, Professor Retzius, has shown that the three lobes of 

 the hemispheres of the human brain, are developed at different 

 periods; the anterior being formed during the second and third 

 months, the middle lobes between the end of the third and begin- 

 ning of the fifth month, and lastly, the posterior lobes are pro- 

 duced. The cerebellum was seen by Von Baer, in the chick, 

 during the fourth day of incubation. It is formed by the meeting 

 of the laminae of the spinal cord anteriorly to the fourth ventricle ; 

 a short canal is, however, left, which passes towards the corpora 

 quadrigemina or optic lobes, the future iter a tertio ad quartum 

 ventriculum. 



Bischoff has demonstrated that, at a very early period, nervous 

 matter is formed along the inner surface of the lips of the primi- 

 tive groove. These two masses of nervous matter gradually 

 approximate, and thus a tube is produced, the walls consisting of 

 nervous matter while the central cavity, after contracting, be- 

 comes the canal of the spinal cord. The upper portion forms the 

 thin dilatations before described ; while at the opposite end is seen 

 a lancet-shaped depression, the future cauda equina, or sinus 

 rhomboidalis in birds. 



Development of the Organs of Vision and Hearing. According to 

 Mr. Gray, the eye of the chick is first seen about the thirty-third 

 hour of incubation, in the form of a protrusion from the anterior 

 vesicle, which corresponds to the cerebral lobes, and may be 

 called the optic vesicle. This view agrees with that of Baer ; but it 

 does not accord with the observation of Wagner or Huschke. The 

 latter observer states, that the eye is developed from a protrusion 

 of the vesicle of the third ventricle from the deutencepJialic en- 

 largement. The retina is a vesicular body which communicates 

 with the cavity of the brain through the hollow, tubular optic 

 nerve. These points may be observed in the chick during the 

 second day of incubation. Bischoff and Mr. Gray have been 

 unable to confirm the statements of Huschke, with reference to 

 the doubling-in of the retina to form two layers. The latter 

 observes that the fibrous lamina and Jacob's membrane are not 

 developed until after the cellular layer of the retina is formed.* 

 * Phil. Trans., 1850. 



