20 ORGANIZATION AND CELL-LINEAGE OF ASCTDIAN EGG. 



the surface of the egg and the chorion, are sometimes carried down with the stream- 

 ing protoplasm to the lower pole of the egg, where they are crowded together and 

 heaped up in the perivitelline space (fig. 3, et seq.). While this flowing is most 

 active, streamers of yellow surface protoplasm may be seen radiating toward the 

 upper pole. The yellow protoplasm thus carried to the lower pole collects into a 

 deep orange-yellow spot which surrounds the sperm nucleus (figs. 4-6); it frequently 

 forms a prominence at the lower pole which recalls the polar lobe of the eggs of 

 annelids and mollusks. The clear nuclear protoplasm also flows to the lower pole, 

 where it lies beneath the yellow disk or spot and is visible around its periphery (figs. 

 4-6). The yellow protoplasm then gradually spreads again until it covers most of 

 the lower hemisphere (figs. 6-10). Then the sperm nucleus moves to one side of this 

 yellow cap, and a large part of the yellow protoplasm is drawn over with it until it 

 forms a yellow band or crescent, in the middle of which the sperm nucleus lies. 

 This crescent lies just below the equator of the egg and its middle point marks 

 the posterior pole of the future embyro. while its two horns reach forward about 

 half-way around the egg to the middle of the right and left sides. 



b. Localisation of Clear Protoplasm and Yolk. 



At the same time that the yellow protoplasm is being formed into a crescent 

 and moved up toward the equator on the posterior side of the egg, the clear proto- 

 plasm which surrounds the sperm nucleus and aster is also drawn entirely away 

 from the lower pole to the posterior side of the egg and thence up to the equator 

 (figs. 82-92). Up to this time the sperm nucleus and the clear and yellow proto- 

 plasm have remained near to the egg surface ; finally, after the meeting of the germ 

 nuclei near the posterior pole of the egg, these nuclei and the clear protoplasm sur- 

 rounding them move inward to the center of the egg, while the yellow protoplasm 

 is largely left at the surface. 



When the clear and yellow protoplasm are withdrawn from the upper pole the 

 gray yolk is there exposed (figs. 4, 5, 11). After the protoplasm moves up to the 

 posterior pole the yolk is exposed over the entire egg, except for the area of the 

 yellow crescent and a narrow line of clear protoplasm, which comes to the surface 

 just above the crescent (figs. 13-18). 



In sections, small spherules which probably represent the yellow granules 

 of the peripheral layer of protoplasm, may be seen heaped up around the entering 

 sperm (fig. 74), this aggregation corresponding to the yellow spot of the living egg 

 (fig. 6). This massing of the yellow spherules is most marked, while the sperm 

 head lies in the peripheral layer ; when it passes through this layer into the deeper 

 layer of clear protoplasm the yellow spherules again spread out into a flattened disk, 

 as shown in figures 75 and 80, which correspond to figures 7 and 8 of the living- 

 egg (Plate I). Later, when the sperm nucleus moves to the posterior pole and the 

 yellow protoplasm is drawn over to that side to form the crescent, sections show 

 that this crescent does not lie entirely on the surface, but that it extends for some 

 distance inward toward the sperm nucleus (figs. 87, 90, 92). 



