1186 



EMBRYOLOGY. 



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

 appendages, consisting of the hairs, nails, sebaceous and sweat glands, are devel- 

 oped from the epiblast, while the corium or true skin is of mesoblastic origin. 

 About the fifth week the epidermis consists of two layers of cells, the deeper one 

 corresponding 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 fetal life, and this desquamated epidermis, 

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



Antihelix. 



Helix. _ < 



Tragus.-' 



Helix. 



Mandible. 



.Helix. 



.'Antihelix. 



Antitragus. 



Tragtis. 



Lobule. 



Lobule 



Antitragus Mandible. 



FIG. 741. Left ears of human embryos, estimated at thirty-five and thirty-eight days respectively. 

 (After His.) 



skin is smeared during the last three months of fetal 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 solid downgrowths 

 of the deeper layer of the epidermis, which then become inverted by papillary 

 projections from the corium. About the fifth month, the fetal hairs (lanugo) 

 appear, first on the head and then on the other parts ; they drop off after birth, 

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

 and sebaceous glands is formed from the epiblast, while the connective tissue and 



Forebrain. 



Aortic bulb.-. 



Auricle. -- 



-- Optic reside. 



Ventricle. 



Omphalo-mesen- 

 teric veins. 



FIG. 742. Head of chick embryo of about thirty-eight hours' incubation, viewed from the ventral surface. 

 (From Duval's Atlas d' Embryologie.) 



blood-vessels are derived from the mesoblast. The mammary gland is also formed 

 partly from mesoblast and partly from epiblast its blood-vessels and connective 

 tissue being derived from the former, its cellular elements from the latter. 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 



