THE POND SNAIL. 159 



the first groove, and deepens until the two cells are divided 

 into four. Then an equatorial groove at right angles to 

 both first and second grooves divides the four cells into 

 eight. This last division is an unequal one, however ; 

 for the groove is not at the equator of the sphere, but 

 nearer the formative pole. It therefore pinches off four 

 small cells at the formative pole, and leaves four large 

 ones, composing the greater part of the sphere. 



It must be observed here that these grooves are but the 

 external evidence of processes going on inside. They are 

 the result, and not the cause, of the division. The cell 

 material separates into two portions ; and the two portions, 

 moving apart, leave grooves between. 



After the eight-cell stage has been reached, the cells 

 divide with less regularity. The small, active cells, of 

 nearly clear protoplasm, at the formative pole, divide 

 very rapidly ; the large, yolk-encumbered cells divide very 

 slowly, so that the later stages in segmentation are ob- 

 scured, and sixteen-cell, thirty-two-cell, sixty-four-cell, 

 etc., stages are not discoverable. But the one-cell, two- 

 cell, four-cell, and eight-cell stages are very plain, and 

 easily followed ; and these illustrate the process of seg- 

 mentation. 



II. Later Stages. Two later stages, which are com- 

 mon to most of the higher animals, may be observed in 

 the development of the snail. These are : 



1. The Uastula stage, in which the segmentation pro- 

 cess results. In this stage there is a hollow sphere of 

 cells, its walls a single layer. The Uastula is not quite 

 spherical, and its walls are not of uniform thickness, 

 being formed of a few large cells on one side, and of 

 more numerous small ones on the other. 



2. The gastrula stage, in which one side of the hollow 

 sphere has become pushed in, as it were ; the other side 

 extending its edges, forming a cup-shaped depression. 



