26 



HUMAN ANATOMY. 



THE FUNDAMENTAL EMBRYOLOGICAL PROCESSES. 



Shortly after the appearance of the primitive streak — a structure, it will be 

 remembered, which is transient and only indirectly takes part in the formation of 

 the embryo proper — a series of phenomena mark the earliest stages of the future new 

 being. These changes are known as the fioidamental embryological processes, and 

 result in the formation of the neural canal, the notochord, and the somites. While 

 described for convenience as separate processes, they progress to a great extent 

 simultaneously. 



Fig. 25. 



^ctoblast 



Mesoblast 



Transverse section through cephalic end of pritnitive streak of very young rabbit embryo. X 100. 



The Neural Canal. — The earliest indication of the embryo consists in the • 

 appearance of two slightly diverging folds (Fig. 27), enclosing the anterior end oi 

 the primitive streak, which are produced by a local proliferation and thickening 

 of the ectoblast. These are the medullary folds and mark the beginning of the 

 formation of the nettral canal, from which the great cerebro-spinal nervous axis, 

 together with its outgrowths, the peripheral nerves, is derived. The medullary folds 

 at first border a shallow and widely open furrow (Fig. 28), the medidlary groove ; 



Fig. 26. 



C 



B 





/i. 



''' '"^^iO^'b 



ft^^. 



m 



Blastula and gastrula stages in the development of amphioxus, drawn from the models of Hatschek. X 260 

 A, blastula composed of single layer of cells surrounding segmentation cavity; ec, en, respectively ectoblastic and 

 entoblastic areas, i?, beginning invagination of entoblastic area {en). C, completed gastrula; ^c, <f«, ectoblast and 

 entoblast ; tn, mesoblast cell ; b, blastopore, leading into archenteron. 



the latter becomes rapidly deeper and narrower as the medullary folds increase 

 in height and gradually approach each other. The approximation of the folds 

 (Fig. 29) and subsequent fusion take place earliest at some distance behind the 

 cephalic end of the groove, at a point which later corresponds to the upper cervical 

 region of the spinal cord. 



After the closure of the groove and its conversion into the medullary canal 

 (Fig. 32), the thickened and invaginated ectoblast forming the lining of the neural 

 tube becomes separated from the outer layer of the embryo by the ingrowth of the 



