ELEMENTS OF THE CENTKAL NERVOUS SYSTEM. 



499 



skin 



sensory 

 cell 7 



intercalated 

 nerve cell 



median groove 



jiefinite patch of ectoderm is set apart to produce the greater part of the nervous 

 j issues for the whole body ; and all except the margins of this area sinks into the 

 |)ody, en masse. 



In one area of ectoderm all the motor nerve-cells develop (Fig. 440, d), in another 



c) only intercalated nerve-cells, in yet another (6) the sensory cells originate ; and 

 jrhe rest forms the epidermis of 



,he skin (a). With our know- 

 edge of the fact that the sensory 



ells were originally distributed 



,hroughout the skin (Fig. 439), 



,he idea naturally suggests itself 



,hat in man also the units of 



,he sensory ganglia might be 



formed in situ in the ectoderm, 



ind that the collection of 



sensory cells in the ganglia 



night possibly be brought 



ibout by the migration of such 



sensory cells inwards, while their 



peripheral processes elongate FIG. 440. DIAGRAM REPRESENTING (IN BLACK) THE LEFT HALF 



,0 permit such migration of the F A TRANSVERSE SECTION OF A 2 MM. HUMAN EMBRYO. 



jell bodies without disturbing Superimposed upon it there is sho*n (in colours) the hypo- 



. . , . thetical primitive arrangement of the nervous elements derived 



iheir Original endings m the from each part of the ectoderm. 



jkin. But there is no evidence 



;.o show, or even to suggest, that such a process takes place in the human embryo. 

 Che facts at our disposal seem to indicate that the sensory cells are derived from 

 iharply circumscribed patches of ectoderm, and that the peripheral processes of 

 /hese cells are distributed to the outlying area of ectoderm beyond them, from 

 vhich the epidermis is eventually formed (Fig. 440). 



At the beginning of the second week the nervous system of the human embryo 

 resented by two thickened plates of ectoderm lying parallel the one to the 



Yolk-sac, 



Neural groove ._ 



Neurenteric canal 

 Primitive streak 



Body stalk 



FIG. 441. THE DORSAL ASPECT OF A VERY EARLY HUMAN EMBRYO (after von Spec). 



other, alongside the median axis of the embryo (Fig. 441), which is occupied by a 



^hallow furrow. 



Upon a diagram (Fig. 440), representing a transverse section through one-half of 

 i an embryo (the uncoloured part), colours corresponding to those employed in 



