54 



EMBRYOLOGY. 



consequence of this the protoplasmic parts of the cell also begin at 

 this time to be divided into halves by means of the process of cleavage, 

 which is already recognisable externally. There is formed at the 

 surface of the egg (fig. 29 A), in a plane passing between the two 

 groups of loops through the middle of the spindle perpendicular 

 to its long axis, a circular furrow, which rapidly cuts deeper and 

 deeper into the substance of the egg, and in a short time divides 

 it into two equal parts. Each of these contains half of the spindle 



Fig. 29 A. Egg of a Sea-ursiiin at the moment of division. 



A circular furrow cuts into the yolk and halves it in a plane which is perpendicular to the 

 middle of the nuclear axis and to the long axis of the dumb-bell. 



B. Egg of a Sea-urchin after its division into two cells. 



In each resultant of the division a vesicular daughter-nucleus has arisen. The radial arrange- 

 ment of the protoplasm begins to become indistinct. 

 Both figures are drawn from the living object. 



with half of the loops, half of the dumb-bell, and a protoplasmic 

 radiation. 



The resulting halves of the egg, still surrounded in common by the 

 vitelline membrane, then closely apply to each other the surfaces 

 resulting from the division, and become so flattened that each one of 

 them forms approximately a hemisphere (fig. 20 E). Internally, 

 however, nucleus and protoplasm enter upon a brief transitory resting 

 stage. There is developed out of the half of the nuclear spindle 

 with its daughter-loops a vesicular homogeneous daughter-nucleus 

 like the first, but in the protoplasm the radial arrangement becomes 

 less and less distinct and at last entirely disappears. 



The egg of the common Maw-worm of the Horse is also a very 

 instructive object for the study of the process of cleavage, as it was 

 for the study of fertilisation, for it allows a still deeper insight into 

 this process. As has already been stated, the egg-nucleus and the 



