8 



Growth in length of the Vertebrate Embryo 



approximately with the centre of this crescent and that of 

 the egg. 



Segmentation as a rule follows the radial type. 



A pit begins on the upper pole of the black hemisphere, and 

 passes round as a furrow dividing the egg into two equal segments, 

 Fig. 4 A, B and E, F. Very soon, after about two hours or less 

 according to the temperature, two furrows start from the centre 

 of this furrow in the black hemisphere and gradually extend so 

 as to divide each segment again into two, so that we have four 

 segments of equal size all lying in the same plane, as in Amphioxus. 



F G 



Fig. 4. Egg of Eana temporaria. 



H 



A, B, C, D, diagrammatic views of segmentation as seen from above; E, F, G, H, 



corresponding stages seen from the side. 



If we compare several views of the lower pole we shall find 

 that there are several varieties of mode of meeting of the furrows 

 at the lower pole. Also we shall notice that the spot where the 

 furrows meet is not in the centre of the white hemisphere. 



The next set of furrows is horizontal. These may be seen 

 commencing from the edges of the two previous furrows and 

 eventually meeting one another, so cutting the four segments into 

 eight. 



In comparing this stage with Amphioxus we note that it 

 corresponds to the third division plane in the radial type of 

 Amphioxus, but is much nearer to the upper pole. There is thus 

 a greater difference between the micromeres and macromeres. 



