300 



HISTOGENESIS 



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. 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 Hne of the embryo. This is the neural plate (Fig. 

 304 A , B) which is folded to form the neural groove (Figs. 77 4 and 78) . The edges 



Neural groove Neural plate 



Neural groove 



Neural plate 



Ectoderm 



Neural groove 





•Neural tube 



Neural tube 



Neural cavity 



D 



Fig. 304. — Four sections showing the development of the neural tube in human embryos. A , An early 

 embryo (Keibel); B, at 2 mm. (Graf Spec); C, at 2 mm. (Mall); D, at 2.7 mm. (Kollmann). 



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 con- 

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

 nected with it, are differentiated the nervous tissues, with the single exception of 

 the nerve cells and fibers of the olfactory epithelium. 



The Differentiation of the Neural Tube. — The cells of the neural tube dif- 

 ferentiate along two Hnes. There are formed: (1) nerve cells and fibers, in which 

 irritabihty and conductivity have become the predominant functions; (2) neu- 

 roglia cells a,nd fibers which constitute the supporting or skeletal tissue peculiar to 



