TEE FCETUS. 



Article II.— Development of the Various Organs in the Animal 

 Economy. 



Development of the Nervous System. 



We will at first glance at the development of the nervous centres — the Irain 

 and spinal cord ,■ then their peripheral parts — the tierves. 



1. First differentiations of the neuraxis. — The trace of the central nervous 

 system or neuraxis, resides, or has been seen, in the primitive groove, resulting 

 from an inflection of the thickened ectoderm (Fig. 551). The complete invagina- 

 tion of the ectoderm brings about the formation of a tube— the neural canal 

 (Fig. 552). From the thirty-sixth hour, the medullary laminae are close to 

 each other in front in the middle line, but are not yet fused together. From 



Fig. 567. 



TRANSVERSE SECTION OF THE BODY OF AN EMBRYONIC DUCK, THREE DAYS OLD. (AFTER BALFOUR.) 



om., Amnion ; so., somatopleure ; sp., splanchnopleure ; wd, Wolffian canal ; st, segmentary tube ; 

 ca.v,, cardinal vein ; m.s., muscular lamina, or plate ; sp.g., spinal ganglion ; sp.c, spinal cord ; 

 ch., notochord ; ao., aorta ; hy, endoderm. 



the fortieth hour, the neural canal is closed as far as the rhomloidal sinus — the 

 lozenge-shaped space where the primitive groove is widest. 



Towards its anterior part, the neural canal shows three successive dilatations 

 — tracts of the brain, and constituting the three primitive cerebral vesicles. The 

 remainder of the neural canal forms the spinal cord. 



2. Development of the brain.— The cerebral vesicles— at first three in number, and 

 distinguished as anterior, middle, and posterior — soon undergo an increase to five. 



The anterior cerebral vesicle throws out two hollow prolongations which 

 become the cerebral hemispheres, and now constitute the anterior brain. On the 

 other hand, the anterior cerebral vesicle itself becomes the vesicle of the optic 

 thalami, or intermediate brain (see Fig. 568). 



