I.] 



SEGMENTATION. 



23 



germinal disc of a hen's egg differs materially from the 

 segmentation of the entire ovum of an amphibian, the former 

 may briefly be described here. 



Viewed from above, a furrow is seen to make its appear- 

 ance, running across the germinal disc and dividing it into 

 two halves (Fig. 5, A). This primary furrow is succeeded 

 by a second at right angles to itself. The surface thus 

 becomes divided into four segments or quadrants (Fig. 5, B). 

 Each of these is again bisected by radiating furrows, and 

 thus the number of segments is increased from four to 

 eight (it may be seven or nine). The central portion of 

 each segment is then, by a cross furrow, cut off from the 

 peripheral portion, giving rise to the appearance of a number 

 of central smaller segments, surrounded by more external 

 elongated segments (Fig. 5, C). 



Division of the segments now proceeds rapidly by means 

 of furrows running apparently in all directions. And it is 



FIG. 6. 



SURFACE VIEW OF THE GERMINAL Disc OF A HEN'S EGG DURING THE LATER 

 STAGES OF SEGMENTATION., (Chromic Acid Preparation.) 



At c in the centre of the disc the segmentation masses are very small and 

 numerous. At 6. nearer the edge, they are larger and fewer; while those at 

 the extreme margin a are largest and fewest of all. It will be noticed that the 

 radiating furrows marking off the segments a do riot as yet reach to the extreme 

 margin e of the disc. 



The drawing is completed in one quadrant only; it will of course be under- 

 stood that the whole circle ought to be filled up in a precisely similar manner. 



