HISTOGENESIS OF THE NERVOUS TISSUES 



299 



digit by the development of new nail substance in the region of the nail 

 fold. The stratum corneum and the epitrichium of the epidermis for a 

 time completely cover the nail matrix and are termed the eponychium 

 (Fig. 303 C}. Later, this is thrown off, but a portion of the stratum 

 corneum persists during life as the curved fold of epidermis which adheres 

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

 base (proximally) , is shifted distally over the nail bed, and projects at the 

 tip of the digit. 



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

 ing 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. 



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. 304 A, B) which is folded to form the neural groove 



Neural groove Neural plate 



Neural groove Neural plate 



Ectoderm 



Neural groove 



Neural tube 



Neural tube 



Neural cavity 



FIG. 304. Four sections showing the development of the neural tube in human embryos. 

 A, An early embryo (Keibel); 5, at 2 mm. (Graf Spee); C, at 2 mm. (Mall); D, at 2.7 mm. 

 (Kollmann). 



(Figs. 77 A and 78). The edges of the neural plate come together and form 

 the neural tube (Fig. 304 C, D). The cranial portion of this tube enlarges 

 and is constricted into the three primary vesicles of the brain (Fig. 324). 

 Its caudal portion remains tubular and constitutes the spinal cord. From 

 the cells of this tube, and the ganglion crest connected with it, are differ- 



