58 



DEVELOPMENT OF THE SPINAL COED. 



termed myelospongium, and the cells by which it is formed are spoken of as spongio- 

 Masts (His) (fig. 61). 



Between the inner ends of the columnar epithelium cells or spongioblasts there 

 is seen at a comparatively early period (four and five weeks in the human embryo) 

 a number of rounded cells, with a considerable amount of clear protoplasm, forming 



Fig. 61. MYELOSPONGIUM FROM SPINAL CORD OF THREE AND A HALF WEEKS HUMAN EMBRYO (His) 7 ^'. 

 Fig. 62. INNER ENDS OF SPONGIOBLASTS WITH GERMINAL CELLS, g, BETWEEN THEM ; FROM 



SPINAL CORD OF HUMAN EMBRYO (His). 



Fig. 63. INNER ENDS OF SPONGIOBLASTS (Sp) ; A GERMINAL CELL (</) AND TWO TRANSITIONAL 



CELLS (2V) FROM SPINAL CORD OF HUMAN EMBRYO (His). 



Fig. 64. THREE NEUROBLASTS, EACH WITH A NERVE-FIBRE PROCESS GROWING OUT BEYOND THE 



BASEMENT MEMBRANE OF THE EMBRYONIC SPINAL CORD (His). 



an interrupted layer in this innermost zone. Their nuclei are mostly in one stage 

 or another of karyokinesis (fig. 62). .They are termed by His the germinal cells, and 

 according to him they give origin to the cells next to be described. 



The third kind of cell (neuroblast) met with in the cord of the early embryo is 

 one with a relatively large oval nucleus, and little protoplasm, but with a tapering 

 protoplasmic prolongation directed outwards towards the surface of the cord. These 

 cells are found in groups, at first only in or near the layer of germinal cells (fig. 63), 

 but subsequently in the outer layers (fig. 64). The prolongations are the com- 

 mencements of the nerve-fibres, and they mostly converge either straight or with 

 an arcuate course towards what will subsequently be the place of exit of the fibres of 

 the anterior roots. 



The outermost layer of the embryonic cord after the differentiation of the 

 various kinds of cells above described is free from nuclei, and is composed of the 

 partly reticulated, partly radially arranged external or attached extremities of the 



