Figure 26. In A is shown the embryo of a salamander that 

 is just beginning to form the first two cells. In B the second 

 plane of cleavage is just beginning and is at right angles to the 

 first. This is termed the four-celled stage. C, D, E, and F, 

 later segmentation stages of this embryo. Notice that the cells 

 are gradually becoming smaller. G shows an early stage in the 

 development of the central nervous system, a stage which occurs 

 much later than the one shown in F. In H the edges of the 

 ridges which have thickened to form the nervous system are draw- 

 ing together; and in I they are united except in the brain region. 

 At this stage it is easy to distinguish for the first time the cells 

 which are to differentiate into the most highly specialized cells in 

 the body of animals, the nerve cells. J, L, and K represent 

 three stages of the embryo after the nervous system has been 

 formed. In j the body of the embryo is forming on the surface 

 of the yolk. L, ventral view to show the formation of the mouth. 

 K (upside-down in the cut). The tufts arising from the neck 

 region are gills and just back of them the front limbs are ap- 

 pearing. On the lower surface of the head is seen a small pit; 

 this is part of the mouth. Just above this pit can be seen a faint 

 circle which is the eye. Better seen in J. Photographs by B. G. 

 Smith. 



