768 



DEVELOPMENT OF THE OKGANS OF SENSE. 



to the deepest pcart of the intruded mesoblast. The vessels of this membrane are 

 svipplied by a branch of the central artery of the retina, which passes foi"n-ards in 

 the axis of the globe, and breaks up at the back of the lens into a brush of rapidly 

 subdividing twigs. The forepart of this tunic, adherent to the pupillary margin 

 of the iris, forms the jnipUhir// mcmhrane by which the aperture of the pupil is 

 closed in the middle periods of foetal life. In the human subject, the whole tunic, 

 to<^ether with the artery which supplies it, becomes atrophied, and is lost sight 

 of before birth, but in some animals it remains apparent for a few days after 

 birth. According to Kolliker, the anterior chamber expands only a short time 

 before birth by the intervention of the aqueous humour between the iris and 

 cornea. 



The eyelids make their appearance as folds of integument, subsequently to the 

 foi-mation of the globe. "When they have met together in front of the eye, their 

 edges become closely glued together ; and they again open before birth. 



The lachrymal canal may be regarded as a persistently open part of the fissure 

 between the lateral frontal process and maxillai-y lobe of the embryo. 



The iii'st discovery of the mode of development of the eye as it is now generally 

 understood was made by Huschke in 1832, and was published in Meckel's Archiv 

 for that year. In addition to the vaiious systematic works on Development 

 previously quoted, the reader is refen-ed to the following, viz., Lieberkiihn, Uber 

 das Auge des Wirbelthier-enibryo, 1872 ; and Julius Arnold, Beitrage zur Entwick. 

 de^ Auges, Heidelberg, 1874. 



DEVELOPMENT OF THE EAR. 



The first origin of the organ of hearing as an involuted follicle from 

 the superficial epiblast of the head, constituting the primary auditory 



Fig. 569. 



Fig. 569. — Outlines showing the Formation of the External Ear in the Fietus. 



A, head and upper part of the body of a human foetus of about four weeks (from 

 nature). \' Four branchial plates (the first, forming the lower jaw, is marked 1), and 

 four clefts are shown ; the auditory vesicle (a), though closed, is visible from the tran- 

 sparency of the parts, and is placed beliind the second branchial plate. 



B, the same parts in a human fcetus of about six weeks (from Ecker). \ The third 

 r.nd fourth plates have nearly disappeared, and the third and fourth clefts are closed ; 

 the second is nearly closed ; but the first (1') is somewhat widened posteriorly in con- 

 nection with the formation of the meatus externus. 



C, human fcetus of about nine weeks (from nature). \ The first branchial cleft is 

 more dilated, and has altered its form along with the integument behind it in connection 

 witli the formation of tlie meatus externus and the auricle. 



or otic vesicles, has already been shortly described. From numerous 

 cbservations there is now no doubt that both in birds and mammals 



