ON THP: anatomy op HISTIUOBDELLA HOMARI. 341 



When the amphiaster attains the pi'ophase, it remains in 

 this stage until the egg is fertilised and deposited in the sea- 

 water. If this does not take place, or if the conditions for 

 egg-laying are unfavourable, it apparently remains in this 

 state indefinitely, not making any further progress. 



In one instance I was able to keep a large female under 

 observation for the greater part of a week with the amphi- 

 aster of its largest egg in the prophase. At the end of this 

 time the fibres of the central spindle and the astral rays were 

 as distinct as at first, and showed no evidence of dissolution. 

 It is evidently contact with the sea-water that is necessary 

 to cause the completion of maturation and the extrusion of 

 the polar body. 



The spindle is of considerable size, measuring from 50- 

 60 // from centrosome to centrosome. It can be readily seen 

 in the living egg with the aid of a good hand-lens. As the 

 animal moves and the egg outline is changed by compression 

 against the body-wall, it does not change the position of its 

 main axis with regard to that of the egg. According to 

 Hertwig's well-known law, as the result of the interaction of 

 the nucleus and protoplasm the spindle comes to lie in such 

 a position that its longitudinal axis corresponds with the axis 

 that passes through the greatest protoplasmic mass. In figs. 

 18-20 are shown the position of the amphiasters in the egg 

 as it has undergone change. The axis of the spindle, it will 

 be seen, does not always correspond with that of the main 

 axis of the egg, but on the whole it lies very close to this, 

 and the cytoplasm always shows a tendency to group itself 

 symmetrically about the spindle. I have made a number of 

 observations that seemed to show that the form of the egg 

 does not greatly affect the direction of the spindle-axis. 



In fig. o2 is shown the egg when it has undergone con- 

 siderable pressure in its long axis through contraction of the 

 animal. The spindle shows no appreciable shortening as the 

 result of this pressure. In fig. 18 the egg shows the com- 

 mencement of two furrows running into the cytoplasm, due 

 to compression against the gut. In fig. 32 a small portion 



