252 COMPARATIVE ANATOMY 



travelled a certain distance downwards from the equator 

 towards the lower pole of the egg. In C the lip of the 

 blastopore forms a larger and wider crescent, and has 

 travelled still further down towards the lower pole. The 

 pigmented cells have grown still further over the white cells, 

 leaving a relatively small white area at the lower pole. In D 

 the process has continued further, and the dorsal Hp of the 

 blastopore nearly coincides with the lower pole of the egg. 

 In E the horns of the blastoporic crescent have turned 

 inwards, and embrace the rapidly diminishing white area. 

 In F the horns of the crescent have nearly met ; the whole 

 ovum is covered with pigmented cells except for a small 

 oval area circumscribed by the rim of the blastopore. The 

 dorsal lip of the last named is now coincident with or a little 

 below the lower pole of the egg. These changes occupy some 

 thirteen hours, and during this time the egg, as a whole, has 

 remained quite still and has not undergone any rotation. It 

 is clear then that the dorsal lip of the blastopore has travelled 

 from above downwards through an angle of about 90°. During 

 the next six hours the egg, as a whole, slowly rotates about a 

 horizontal axis in the direction of the point at which the dorsal 

 lip of the blastopore first made its appearance. Consequently 

 in G we find that the dorsal lip has returned some way back 

 along the path by which it moved downward, but this time 

 its change of position is not due to its own movement but 

 to the rotation of the egg as a whole. In point of fact, 

 while the egg is rotating backwards, the dorsal lip of the 

 blastopore is still moving slowly along its original path, and at 

 the same time its sides are contracting towards the centre, so 

 that the white area is restricted to a small patch of yolk-cells 

 circumscribed by the now circular rim of the blastopore. This 

 area is still further diminished in size by the contraction of the 

 blastoporic rim, and in H \h& yolk-cells form a projecting mass 

 known as the blastoporic or yolk-plug. Finally in y the yolk- 

 cells are entirely covered in by pigmented cells, and the blasto- 

 pore forms a minute orifice at the bottom of a slight depres- 

 sion. It should be noted that in consequence of the rotation 

 of the whole egg in a direction the reverse of that along which 

 the dorsal lip of the blastopore travelled, the blastopore in^ is 

 slightly above the point occupied by the dorsal lip in A. 



We must now study the internal changes that have been 



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