1010 OP GENERATION. 



spondence between the progressive stages of development of the Human Cere- 

 brum, and those which we encounter in the ascending scale of Mammalia. 1 



Brain of Human Embryo at twelfth week: A, seen from behind; B, side view; c, sectional view; a, corpora 

 quadrigemina ; Z>, b, hemispheres ; d, cerebellum ; e, medulla oblongata ; /, optic thalamus : g, floor of third 

 ventricle ; i, olfactory nerve. 



1013. The development of the two principal Organs of Sense, the Eye and 

 the Ear, has been made the subject of careful study (in the Chick) by Mr. H. 

 Gray. 3 f h e development of the Eye commences by a protrusion from the 

 posterior part of the anterior cerebral vesicle, representing the " vesicle of the 

 thalami optici," which is at that time hollow ; and the cavity of the protrusion 

 is continuous with that of the vesicle itself, which remains as the " third 

 ventricle/' This protrusion is lined, like the cerebral vesicle, with granular 

 matter, which gradually becomes distinctly cellular, forming a layer of a truly 

 ganglionic character; and whilst this change is taking place, the protrusion 

 increases, becomes pear-shaped, and is at last connected only by a narrow pedicle 

 with the vesicle from which it sprang. This pedicle closes up, so as completely 

 to separate the two cavities; and the one which has been thus budded forth 

 constitutes the rudiment of the eye, whilst the other goes on to form the gan- 

 glionic bodies at the base of the cerebrum, the connecting pedicle becoming the 

 optic nerve, which connects the retina with its ganglionic centre. The spherical 

 extremity of the protrusion is absorbed, and the retina, or vesicular lining, 

 becomes attached to the margin of the lens, which is in the mean time developed 

 in the interior of the cavity, and is at first completely surrounded by the retina. 

 The formation of the coats of the Eye takes place subsequently; the development 

 even of the " fibrous lamina" and of the " membrana Jacobi" of the retina itself, 

 not taking place until after its cellular layer has been very distinctly formed. 

 It is a curious circumstance, and one not very easy to account for, that the de- 

 velopment of the Eye should take place from the Deutencephalic and not from 

 the Mesencephalic vesicle ; as it is in the latter that the proper " optic ganglia" 

 originate, with which the optic nerves come at last to have their principal con- 

 nection, their connection with the " thalami optici" being much less close. 

 The Auditory apparatus takes its origin in a portion of the Epencephalic vesicle, 

 which protrudes on either side, its cavity at first communicating with that of 

 the vesicle, which remains permanent as the " fourth ventricle." As its pro- 

 trusion increases, it becomes elongated and pear-shaped, and is only connected 

 with the central mass by a pedicle whose canal gradually closes up ; the sac thus 

 cut oft 7 becomes the vestibular cavity, and the pedicle the auditory nerve. At 

 first there is no vestige either of cochlea, semicircular canals, or tympanic appa- 

 ratus ; but the sac presents the simple character which it permanently retains 

 in the Cephalopoda and the lower Fishes. Gradually, however, the semicircular 



1 See an account of the observations of Prof. Retzius on the Development of the Cere- 

 brum, in the " Archives d'Anatomie Generale et de Physiologic," 1846. 

 a " Philosophical Transactions," 1850. 



