94 NERVOUS SYSTEM 



the second month this layer has resolved itself into two lamellae a deeper of pol 

 hedral cells, and a superficial of flattened cells, the rudiment of the stratum corneum 

 of. the skin. As development proceeds, both of these layers increase in thickness, 

 and several cellular strata are laid down. By the fifth month the surface cells 

 begin to be shed, and form with the secretion of the sebaceous glands the cheesy 

 layer on the skin of the foetus known as the vernix caseosa. Meantime, the 

 underlying mesenchyme has differentiated into a connective-tissue layer which 

 becomes the corium or true skin, and projections from this develop into the 

 vascular papillae. The epithelium of the glands is formed from the deeper layer 

 of the primitive epidermis. Solid processes grow inwards from this into the 

 mesenchyme ; in these a lumen is developed later. The sweat-glands open on 

 to the surface, but the sebaceous glands are formed in connexion with primitive 

 ingrowths of the epidermis, which give rise to the hairs and their root- sheaths. 

 The surrounding mesenchyme gives rise to the connective-tissue elements of the 

 glands, the sheaths, and papillae of the hairs, and also to the muscular tissue of the 

 arrectores pilce muscles and muscular tissue of the sweat-glands. 1 



The primitive downy hair of the foetus is known as the lanugo. It is partly 

 shed in the later months of pregnancy, and is replaced after birth by permanent 

 hair. 



DEVELOPMENT OF THE NERVOUS SYSTEM. 2 



As has been already described, the whole of the central nervous system takes 

 origin from the thickened walls of a dorsally situated axial groove, subsequently 

 converted into a canal, which runs forwards in front of the primitive streak. The 

 anterior end of this canal becomes enlarged and converted by constrictions into 

 three vesicles, around which the several parts of the brain are formed. These 

 enlargements are known as the primary cerebral vesicles. The rest of the neural 

 canal remains of nearly uniform diameter ; its walls become converted into the 

 substance of the spinal cord, while the cavity itself becomes eventually the central 

 canal of the cord. The walls of the neural groove are of course composed of 

 ectoderm, and it therefore follows that the whole structure of the central nervous 

 system is laid down in that layer, and consists in the main of more or less modified 

 ectodermic elements, except where mesodermic tissues subsequently penetrate 

 into it, conveying blood-vessels into its substance. The same is in all probability 

 true for all the nerves of the body, cranial and spinal. 



Histogenesis of nervous tissue and peripheral nerves. Before 

 entering on a description of the phases through which the neural canal passes as 

 the brain and spinal cord take form, it will be convenient to give a general 

 account of the changes by which the ectoderm of the wall of the canal is converted 

 into nervous tissue. The changes are in essence the same in all parts of the 

 tube, and in all vertebrates. 



The neural plate consists at first of a layer of columnar epithelium. The 

 divisional planes between the cells are rather ill-defined. The outer ends of the 

 elements are composed of granular protoplasm, the inner ends are finely striated. 

 The nuclei are in several ranges, placed at different levels ; the innermost are 

 aJmost without exception in one phase or another of karyokinesis (fig. 130). 

 Proliferation is thus taking place on the free surface (afterwards the inner surface 

 of the closed neural canal), and through all successive phases this zone of dividing 

 nuclei persists until a certain stage of development is reached. It is therefore 



1 Some authorities derive the muscular tissue of the sweat-glands from the ectoderm. 



2 The literature concerning the morphogenesis of the central nervous system in the Primates will 

 be found in Ziehen's article in Hertwig's Handbuch II. Part III. p. 386 seq. ; that concerning histo- 

 genesis, ibid. p. 434 seq. For the literature of the development of the peripheral nervous system 

 see Neumayer, ibid. p. 621 seq. References to more recent papers mentioned in the text are given in 

 the footnotes. 



