THE HISTOGENESIS OF THE NERVOUS TISSUES 



3°9 



matrix and is termed the eponychium (Fig. 296 C). Later, this is thrown off but 

 a portion persists during life as the curved fold of the epidermis which adheres to 

 the lunula of the adult nail. During life the nail constantly grows at its base 

 (proximally), is shifted distally over the bed of the corium, and projects at the tip 

 of the digit. The corium distal to the lunula takes no part in the development 



■-Sole plate 



— Nad 



- Lunula 



Eponychium 



~"Nail matrix 

 * -Nail fold 



Nail bed 



Fig. 296. — Figures showing the development of the nail. A and B, in surface view; A in a 4 

 cm., B, in a 10 cm. fetus; C, longitudinal section through the nail anlage of a 10 cm. fetus. X 24 

 (Kollmann's Handatlas). 



of the nail substance and its surface of contact with the nail is thrown into parallel 

 longitudinal folds. These folds produce the longitudinal ridges of the nails. 



The nails of man are the homologues of the claws and hoofs of other mammals. During 

 the third month thickenings of the integument over the distal ends of the metacarpals and 

 metatarsals become prominent. These correspond to the touch-pads on the feet of clawed 

 mammals. Similar pads are developed on the under sides of the distal phalanges. 



THE HISTOGENESIS OF THE NERVOUS TISSUES 



The primitive anlage of the nervous system consists of the thickened layer of 

 ectoderm along the mid-dorsal line of the embryo. This is the neural plate (Fig. 

 297 A, B) which is invaginated to form the neural groove. The edges of the 

 neural plate come together and form the neural tube (Fig. 297 C, D). The 

 cranial portion of this tube enlarges and is constricted into the three primary 

 vesicles of the brain (Fig. 306). Its caudal portion remains tubular and con- 

 stitutes the spinal cord. From the cells of this tube and the ganglion crest con- 



