Wyman.] 



250 



[June 5, 



when the spinal groove closes up by the union of the dorsal laminae, it 

 does so, as Reichert has shown, by the union of its lips, first in the 

 middle portion, and then gradually in a symmetrical manner towards 

 either end. Fourthly, in the four classes of vertebrates the first traces 

 of the vertebral segments are to be found in three or four pairs of 

 plates which appear on either side of the primitive axis midway be- 

 tween the two ends, from which region they ai-e multiplied and ex- 

 tended forwards to the head, and backwards to the tail, by the devel- 

 opment of new plates, thus lengthening the column in a symmetrical 

 manner (Figs. 3, 4). The ossification of the bodies of the vertebrae 



Fig. 3. 



Fir. 4. 



takes place in the same order, beginning in the middle and extending 

 in either direction. During the first half of foetal life, and even be- 



