426 EMBRYOLOGY OF THE BRAIN AND SPINAL CORD. 



medial fillets and form the pyramids of the medulla oblongata. 

 The form of the medulla is completed two months later (the sixth 

 month) by the appearance of the olivary bodies (Fig. 93). 



SPINAL CORD. 



That portion of the neural tube which is situated behind the 

 metencephalon is the embryonic spinal cord (Figs. 16 and 118). 

 It is of nearly uniform size from cephalic to caudal end. The 

 lumen of the neural tube in this region is at first large and ellip- 

 tical in shape. Later, at the sixth week, it has a diamond shape, 

 the acute angles of the diamond being formed by the roof-plate 

 and floor-plate of the canal; it is lined with columnar ciliated 

 cells (Fig. 117). As the walls thicken the canal is contracted 

 more and more until it reaches the capillary size of the adult cord. 

 The canal is continuous with the fourth ventricle above and dilates 

 to form the ventriculus terminalis in the filum terminale internum 

 (Fig. no). The spinal part of the neural tube forms the whole 

 substance of the spinal cord, with the exception of the great motor 

 tracts that grow into it from the brain, and the sensory tracts and 

 fibers that enter it from the spinal ganglia. At the sixth week 

 of embryonic life the anterior and posterior roots of the spinal 

 nerves are clearly seen; they are horizontal in direction. The 

 cord extends to the fourth mesoblastic somite of the coccyx, when 

 the somites are first laid down; but, as no neuroblasts are developed 

 by the three lower segments of the human cord, they form a con- 

 nective tissue strand, the primitive filum terminale (McMurrich). 

 The cord occupies the entire length of the spinal canal until the third 

 month, when the caudal end begins to recede and the filum ter- 

 minale to lengthen. It reaches only to the third lumbar verte- 

 bra at birth and, in the adult, but to the lower border of the first 

 lumbar vertebra. With the rapid growth of the spinal column, 

 the roots of the lumbar, sacral and coccygeal nerves and the filum 

 terminale become greatly elongated and form the cauda equina. 



Meninges. The investing mesoblast of the neural tube devel- 

 ops the meninges of the spinal cord. 



Zones (Figs. 117 and I27A). By the sixth week of intrauterine 



