ORGANIC EVOLUTION 



193 



Development. — The way of access to intelligent compre- 

 hension of the cellular structure of the salamander lies 

 through the study of its development. 



The salamander begins life as a single cell, the result of 

 fusion of egg and sperm. It is a very large cell, because 

 distended with yolk and enveloped by a thick gelatinous 

 envelope; but the protoplasmic part, the living part of it, 

 is very small. The protoplasm is not equally distributed 

 through the egg, but is more abundant on the upper pig- 

 mented side. Therefore, the division planes in cleavage 

 start on the upper side, and division is somewhat retarded 

 below by the impeding yolk mass. The cell divides into 

 two cells along a meridional plane and the two divide into 

 four by another meridional plane at right angles to the 

 first; then the four divide into eight by a plane parallel 

 to the equator of the sphere, at right angles to both former 

 planes, not at the equator, but a little nearer the darker 

 upper pole, where it divides the protoplasm more equally. 

 Successive meridional planes, and planes parallel to the 

 equator mark the following divisions into i6-cell, 32-cell, 

 etc., stages which, however, are not traceable farther 

 because of the retardation of division on the lower side, and 

 because of the planes getting ajog at the joints. The result 

 is clearly a blastula, as before — a hollow sphere of cells of 

 small size. The slipping inward from the surface of some of 

 its cells results in its being more than one layer in thickness 

 over part of the upper side, and the retardation of division 

 owing to excess of yolk on the lower side throws the segmen- 

 tation cavity above the middle of the sphere. All these 

 facts are indicated in the accompanying figures. Then 

 gastrulation takes place in a manner that is yet more 

 aberrant. An ingrowth from the outer wall gives rise to 

 endoderm surrounding an arch-enteron as before ; but the 

 ingrowth, impeded by yolk does not result in a widely open 



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