THE NOSE. 125 



tuting its fibrous and vascular layer. With regard to the exact mode of develop- 

 ment of the ossicles of the middle ear there is considerable difference of opinion. The 

 malleus and incus, however, seem to be developed from the proximal end of the 

 mandibular (Meckel's) cartilage, while the stapes seems to have a double origin, 

 its plate being an ossification of the cartilage which fills the foramen ovale in the 

 embryonic condition, while its arch is an ossification of the upper end of the 

 cartilage of the hyoid arch. 



The external auditory meatus is developed, like the pinna, from the soft parts 

 on the posterior margin of the first visceral cleft by an outgrowth of the tissues 

 in this situation. 



Development of the Nose. The olfactory fossae, like the primary auditory 

 vesicles, are formed in the first instance by a thickening and involution of the 

 epiblast, which takes place at a point below and in front of the ocular vesicle 

 (Fig. 92, 2, 3). The thickening appears at a very early period, about the fourth 

 week. The borders of the involuted portion very soon become prominent, in con- 

 sequence of the development of the mid-frontal and lateral naso-frontal plates 

 above spoken of (page 119), which are formed on either side of the rudimentary 

 fossae. As these processes increase the fossae deepen and become converted into 

 a deep channel, Avhich eventually forms the upper part of the nasal fossae that 

 is, the two superior meatuses, the part to which the olfactory nerves are dis- 

 tributed. At this time they are continuous with the buccal cavity, a portion of 

 which forms the lower part, or inferior meatus of the nasal fossae. For as the 

 palatine septum is formed the buccal cavity is divided into two parts, the upper 

 of which forms the lower part of the nasal fossae, while the remainder forms the 

 permanent mouth. 



The soft parts of the nose are formed from the coverings of the frontal pro- 

 jections and of the olfactory fossae. The nose is perceptible about the end of the 

 second month. The nostrils are at about the third month closed by the growth 

 of their epithelium, but this condition disappears about the fifth month. 



The olfactory nerve, as above pointed out, is formed from the anterior cerebral 

 vesicle as a secondary vesicle on its under surface, and it lies upon the involuted 

 epiblast. which subsequently forms the nasal fossae. 



Development of the Skin, Glands, and Soft Parts. The epidermis is produced 

 from the external, the true skin from the middle, blastodermic layer (Fig. 79, 19, 

 20). About the fifth week the epidermis presents two layers, the deeper one cor- 

 responding to the rete mucosum. The subcutaneous fat forms about the fourth 

 month, and the papillae of the true skin about the sixth. A considerable desqua- 

 mation of epidermis takes place during foetal life, and this desquamated epidermis, 

 mixed with a sebaceous secretion, constitutes the vernix caseosa, with which the 

 skin is smeared during the last three months of foetal life. The nails are formed 

 at the third month, and begin to project from the epidermis about the sixth. The 

 hairs appear between the third and fourth months in the form of depressions of 

 the deeper layer of the epithelium, which then become inverted by a projection 

 from the papillary layer of the skin. The papillae grow into the interior of the 

 epithelial layer ; and finally, about the fifth month, the foetal hairs (lanugo) appear 

 first on the head and then on the other parts. These hairs drop off after birth, 

 and give place to the permanent hairs. The cellular structure of the sudorifer- 

 ous and sebaceous glands is formed from the epithelial layer, while the connective 

 tissue and blood-vessels are derived from the mesoblast about the fifth or sixth 

 month. The mammary gland is also formed : partly from mesoblast its blood- 

 vessels and connective tissue ; and partly from epiblast its cellular elements. 

 Its first rudiment is seen about the third month, in the form of a small projection 

 inward of epithelial elements, which invade the mesoblast; from this similar 

 tracts of cellular elements radiate ; these subsequently give rise to the glandular 

 follicles and ducts. The development of the former, however, remains imperfect, 

 except in the adult female. 



Development of the Limbs. The upper and lower limbs begin to project, as 



