Formation of Nervous System. 



131 



must be noted that the brain and the spinal cord arise as a 

 continuous structure, and that at first the brain is only to be 

 distinguished by its wider calibre. 



While the changes in the epiblast are going on, the meso- 

 blast also becomes differentiated. It forms at first a continuous 



Fig. 68. — Surface view of embryo Chick at 24th hour. (From Marshall.) 



HD, head ; nf, neural fold ; vv, vitelline vein ; ng, neural groove ; MS, proto-vertebrse ; 

 AD, margin of area pellucida; PS, primitive streak. 



sheet with the threefold origin already spoken of. The first 

 change to be noted is its splitting into a dorsal and a ventral 

 layer, with the result that a cavity appears between them. 

 This cavity is the eoBlom. The upper layer of mesoblast is 

 known as the somatopleure, the lower as splanchnopleure. 



