DEVELOPMENT OF THE FCETUS. 99 



are derived from the external blastodermic layer. The part of the 

 integument not required to forn: the lens constitutes the envelope of the 

 globe. The latter forms the sclerotica and cornea ; while the epiderm 

 furnishes the epithelium to the latter, which becomes distinct from the 

 sclerotica in about the fourth month. A slit occurs at the lower part of 

 the fibrous envelope of tlie globe ; this is related to the development of 

 the vitreous humour, a prolongation of the derm passing through this 

 slit and entering between the lens and the anterior wall of the secondary 

 ocular vesicle which appeared shortly before. There this prolongation 

 becomes developed and transformed into the vitreous humour, wliich at 

 cue period is surrounded by vessels, but shows none immediately before 

 birth. It exhibits in its centre a transverse canal, which lodges a 

 branch of the arteria centralis of the retina. 



The optic nerve is developed in the pedicle connecting the ocular 

 vesicle with the anterior cerebral vesicle, and the rci'ina is formed by the 

 inner layer of the secondary ocular vesicle ; it extends to the lens in 

 changing its character in front. The choroid coat is constituted by the 

 posterior layer of the ocular vesicle ; it extends as far as the lens, and 

 is then inflected in front of that body to form the iris. The borders of 

 the pupil embrace the vascular envelope of the lens, and the anterior 

 face, as well as this pupil, is covered by a very vascular membrane, the 

 viemhrana i)upillaris ; behind it is the equally vascular covering, the 

 mcmhrana capsulo-pupillaris, that passes through the pupil to the lens to 

 envelop it in a kind of sac that disappears towards the end of gestation. 

 Previous to this time this aperture is very wide ; but as the iris is 

 developed the pupil contracts, and the vessels of the vascular or 

 pupillary membrane diminish in size and number, until at last only a 

 few are seen crossing the transparent membrane. 



The protective and motor apparatus of the eye is gradually developed 

 around the globe. The eyelids are small cutaneous folds which are 

 formed in the first third of uterine life, and grow and unite by their 

 margins until a short time before or after birth, according to the species, 

 when they separate. They are maintained closed by a thin membrane, 

 which disappears in Solipeds, Ruminants, and the Pig, before birth ; but 

 in Carnivora it remains until some days (eight or ten) after that event. 

 So long as the lids are closed, the conjunctiva is only a sac comnmni- 

 cating with the lachrymal canal. The crystalline lens in the fcetus and 

 new-born animal distinctly shows the three septa peculiar to it ; three 

 diverge from each pole at angles of 120'. The lachrynial gland is an 

 appendage of the epithelial layer which is intruded above the globe ; 

 at first compact, it becomes gradually excavated into cavities, from 

 which arise the excretory ducts. 



The auditory apparatus, consisting of the intei*nal ear, auditory nerve, 

 and middle ear, is developed separately. The labyrinth appears in the 

 form of a vesicle which is not in direct relation with the posterior cere- 

 bral cell, but is constituted by a depression of the epidermic layer — tbe 

 auditor]! fossa — that becomes more and more marked until it is finally 

 a closed cavity. At this time the wall of the labyrinth is only a simple 

 epithelial membrane ; but this is soon increased externally by a con- 

 nective membrane which vascularises it, and then gives rise to three 

 layers : an external, which adheres to the epithelium to form the mem- 

 brane of the labyrinth ; an external, that lines the labyrinthic cartilage; 

 and a middle, whose soft embryonic connective tissue disappears and is 

 replaced by a fluid, the perilymph. At the same time that these 



