500 THE JS T EEVOUS SYSTEM. 



Fig. 439 have been placed to indicate the nature of the elements that are known to 

 develop in relation with each area of the ectoderm at a later period in the history of 

 the embryo : 6 represents an area which later will form the crista neuralis, from 

 which the sensory cells will be developed. The peripheral processes of these cells 

 will pass into the skin (a) and their central processes into the area cd, which will 

 become part of the neural tube. In the area c intercalated cells will develop to 

 receive the incoming sensory nerves ; and in the area d the motor nerve-cells (as 

 well as other intercalated cells) will be formed. 



When it is recalled that all the elements of the primitive nervous system of 

 Hydra are modified ectodermal cells, and, moreover, that when the intercalated 

 and motor nerve-cells wander into the deeper tissues the protoplasm of the whole 

 nervous network remains in uninterrupted continuity (Fig. 439), it is instructive to 

 note that in the primitive human nervous system the rudiment of the epidermis 

 of the skin is linked to the medullary plate by the patch of ectoderm from which 

 the sensory ganglia will be formed. 



In the discussion of the inter-relationships of the various constituent elements 

 of the nervous system, there will be occasion to refer to this matter again. 

 But while we are studying Fig. 440 it is important to emphasise the fact that in 

 accordance with the commonly accepted ideas it is taught that the area & becomes 

 completely severed from a and c, and shortly afterwards fibres are budded off from 

 the cells in the area I to form the sensory nerves linking a to c, thus re-establish- 

 ing a connexion which existed a few days earlier. This suggests the possibility 

 that the connexions between these three series of elements may not have been 

 completely sundered during the intermediate phase of development. Early in the 

 second week in the human embryo the axial groove separating the two bands of 

 thickened ectoderm (Fig. 441) that form the medullary plate becomes deepened by 

 the tilting-up of the lateral margins of the two bands. This process becomes 

 accentuated during the next day or two until a deep cleft is formed, the walls of 

 which consist of the thickened ectoderm and the floor of the thinner ectoderm 

 (floor-plate) joining them together. Before the end of the week the dorsal edges 

 of these thickened plates become joined in the region which will develop into the 

 neck; and during the third week the sealing of the lips of the neural groove 

 extends upwards (headwards) and downwards (tailwards), so that the neural tube 

 becomes completely closed by the end of that week. The extreme anterior (head-) 

 end and the dorsal aspect of the caudal extremity of the tube are the last parts to 

 close, the latter being, as a rule, a little later than the former. When the tube is 

 in the stage of being patent only at its two ends, the openings are known as the 

 neuroporus anterior and neuroporus posterior, respectively. 



In the process of closing, the extreme dorsal edge of the medullary plate 

 becomes excluded, in the greater part of its extent, from participation in the 

 constitution either of the neural tube or of the skin, and forms a column of celle 

 lying between the two. This is the neural crest (Fig. 442, A, B, and C ; x and y 

 represent the places where the apparent sundering occurs). 



It is commonly supposed that the neural crests do not extend the whole lengtl 

 of the neural tube. Nevertheless, peculiar ectodermal areas, which ultimately giv< 

 origin to sensory nerves, are found at the junction of the medullary plate with th 

 skin in those regions where the neural crest is supposed to be lacking. At th' 

 extreme anterior end of the neural tube the margins of the anterior neuropor 

 become thickened to form crest-like patches ; but when the tube closes these area 

 do not separate from the skin (at x, Fig. 442, D), as the rest of the neural crest doe; 

 They remain part of the skin and become the olfactory areas, in which sensor 

 cells, precisely like those found in Hydra (Fig. 439), develop. 



A little farther on the caudal side of the olfactory region a very large cresf 

 like mass of ectoderm fails to separate from the medullary plate as it closes, an 

 becomes a constituent part of the neural tube (Fig. 442, E). It develops into tl: 

 optic diverticulum from which the cells of origin of the optic nerve are formed. 



In several other regions sensory nerves originate from cells of ectodermal, ar 

 possibly even entodermal, areas which do not form parts of the neural crest, 

 that term is usually understood. The nerves of hearing and taste are developi 



