256 THE FIFTH DAY. [CHAP. 



portant parts of the cord have been defiuitely es- 

 tablished : 



(1) The anterior and posterior fissures. 



(2) The anterior and posterior horns of grey 

 matter. 



(3) The anterior, posterior and lateral columns 

 of white matter. 



(4) The spinal canal. 



As yet, however, the grey masses of the two sides of 

 the cord only communicate by the anterior grey com- 

 missure, and the white columns of opposite sides do 

 not communicate at all. The grey matter, moreover, 

 still far preponderates over the white matter in 

 quantity. 



By the ninth day the posterior fissure is fully 

 formed, and the posterior grey commissure has also 

 appeared. 



In the centre of the sacral enlargement this com- 

 missure is absent, and the posterior columns at a later 

 period separate widely and form the 'sinus rhomboi- 

 dalis,' which is not, as has been sometimes stated, the 

 remains of the primitive ' sinus rhomboidalis ' visible 

 during the second day. 



The anterior white columns have much inci'eased on 

 this day, and now form the sides of the already deep 

 anterior fissure. The anterior white commissure does 

 not however appear till somewhat later. 



The heart. The fifth day may perhaps be taken 

 as marking a most important epoch in the history of 

 the heart. The changes which take place on that and 

 on the sixth day, added to those previously undergone, 



