THE FCETUS. 



1037 



and which becomes the membrana tympani ; the internal portion is the middle 

 ear and Eustachian tube ; the external portion is the external auditory canal. 



The ossicula auditus appear in a cartilaginous state towards the third month ; 

 they gradually ossify, and have scarcely acquired their definitive volume at birth. 

 The external ear {concha) is developed beneath the integument after the second 

 month. 



3. Olfactory apparatus. — This commences by two depressions of the epider- 

 mic lamina, analogous to the auditory fossa and that of the lens. These two 

 olfactory fossas appear below the ocular vesicles, and become more and more dis- 

 tinct, being margined by small projections which increase their depth. Behind, 

 they communicate with the pharynx. The appearance of the palate separates 

 them in front from the buccal cavity, and from 

 this period the nasal fossae are constituted. They 

 are completed by the development of the bones 

 of the face. 



The olfactory hidbs and nerves are primarily 

 hollow, and joined to the anterior cerebral cell. 

 The nostrils are formed, in the young foetus, by 

 a mass of mucus and epithelium ; towards the 

 middle of gestation they are open. 



4. Gustatory apparatus. — See, subsequently, I 

 the development of the tongue^ in digestive ap- 



AN EMBRYO (HUMAN) OF FOCB 

 WEEKS, ENLARGED ABOUT THREE 

 TIMES. 



a. Vesicle of corpora quadrigemina ; 

 6, vesicle of cerebral hemispheres; 

 c, vesicle of third ventricle ; d, 

 vesicle for cerebellum and medulla 

 oblongata ; e, auditory vesicle ; /, 

 olfactory fossa ; h, liver ; **, 

 caudal extremity. 



5. Tactile apparatus. — The skin and its de- 

 pendencies. — The skin is developed at the expense 

 of the epidermic lamina of the middle layer of 

 the blastoderm. The cutaneous laminfe — by the 

 modification of their elements — form the derma, 

 in which the blood-vessels are very apparent at 

 the third month. In the epidermis, the mucous 

 and the horny layers are soon distinguished ; in 

 the first, the pigment is visible at the commence- 

 ment of the fifth month in the larger Quad- 

 rupeds. When the foetus increases in volume, the epidermis exfoliates, and its 

 debris floats in the liquor amnii. 



In the third month, the hairs are seen in the foetus of the Mare and Cow ; 

 they appear at first on the eyebrows, lips, and the joints of the limbs ; at the sixth 

 or seventh month they cover the body. They may be shed and renewed before 

 birth. They are developed in a prolongation of the epidermic lamina, which is 

 embedded in the substance of the derm ; it is shaped like a httle bottle, and is 

 composed of a mass of cells ; in its centre, these cells are modified and collected 

 together to form a small cone, the base of which covers the growing papilla. This 

 cone becomes elongated, touches the surface of the epidermis, doubles under the 

 effort to push through it, and finally makes its exit, after which it can grow freely. 



The sebaceous and sudoriparous glands are developed in the same manner, 

 towards the middle period of uterine existence. 



" The horny productions — the claics, hoofs, ergots, chestnuts — begin to show 

 themselves early. Towards the end of the second month, in the fcetus of the 

 Cow, there is perceived, at the extremity of each limb, a small pale, translucid, 

 conical tubercle, which is the rudiment of the hoof. At the commencement of 



