The Development of Phascolosoma. 



95 



Fh. gouldii than in Ph. vulgare, a fact which is propably due to the 

 larger amount of yolk present in the egg- of the former. 



Third Cleavage and Eight-cell Stage. 



About twenty minutes after the establishment of the four-cell 

 stage, spindles, pointing upward in dexiotropic positions, indicate a 

 preparation for the eight-cell stage (Fig. 21). In A, B and C they 

 lie slightly nearer the vegetative than the active pole; in D, however, 

 nearer the active pole. 



Fig. B. 

 8-cell stage in Phascolosoma gouldii. 1. side view. 2. active pole. 320 : J . 



I believe that a redistribution of the protoplasm, and ac(îordingly 

 of the yolk, takes place in J_, B and C at this time, for whereas 

 the nuclei in the resting four-cell stage lie nearer the active pole, 

 the spindles which arise from them now move toward the passive 

 pole. This shifting obviously indicates that the protoplasm, which, 

 during the maturation and fertilization is concentrated at the forma- 

 tive pole, moves slightly towards the other, while a certain amount 

 of the yolk-laden peripheral protoplasm make a compensatory move- 

 ment towards the active pole. This readjustment is clearly in- 

 dependent of external influences, as, for example, gravity, for it 

 takes place in whatever position the %gg may be placed during 

 development. As to its meaning, all that can be asserted at present 

 is that the ç,^g has an inherited tendency to form larger blasto- 

 meres at the active than at the passive pole in quadrants A, B 

 and C. 



