ORGAN-FORMING SUBSTANCES IN EGGS OF ASCIDIAN.S. 211 



germ nuclei and amphiaster move to the center of the egg and 

 the clear protoplasm goes with them. Finally in the telophase 

 of the first cleavage the clear protoplasm moves into the upper 

 (animal) hemisphere, while the yellow and gray protoplasm are 

 situated in the lower (vegetal) hemisphere. 



The first cleavage cuts through the middle of the crescent, the 

 clear protoplasm, and the yolk, the division of all the egg sub- 

 stances being bilaterally symmetrical. At the close of tliis cleavage 

 each of these substances occupies its definitive position in tJic egg 

 (Photos 6-S). Tlic upper clear half of the egg gives rise to 

 ectoderm ; the crescent of yelloiv protoplasm surrounds the posterior 

 side of the egg jnst below the equator and is later transformed into 

 the muscle and mcscncliyme cells of the larva ; the gray protoplasm 

 occupies the remainder of the loiter /temisp/icre and gives rise to the 

 cndodenn, to t/ie chorda, and to the neural-plate. Two areas are 

 distinguishable in the gray substance, though I had failed to see 

 them until my attention was called to them by the photographs; 

 the posterior part of the gray material lying in front of the 

 crescent and extending some distance anterior to the vegetal pole 

 is deeper in color and contains more yolk than the anterior 

 portion ; the latter forms a light gray crescent around the ante- 

 rior border of the vegetal hemisphere, just as the yellow proto- 

 plasm forms the yellow crescent around its posterior border. 

 The 2 -cell stage when seen from the right or left sides (right 

 side in Photo 8) shows all of these areas distinctly, the yellow 

 crescent at the posterior pole (very dark in the photograph), the 

 deep gray material anterior to this, and the light gray crescent 

 occupying the anterior third of the vegetal hemisphere and ex- 

 tending a little above the equator, while the clear protoplasm is 

 located chiefly in the upper hemisphere. The dark gray portion 

 of the vegetal hemisphere gives rise to the endoderm of the larva, 

 the light gray crescent to the notochord and neural plate. 1 All 

 the principal organs of the larva in their definitive positions and 

 proportions are lie re marked out in the 2 -cell stage by distinct 

 kinds of protoplasm ! 



1 It is an interesting question whether the chorda-neural-plate crescent of the 

 ascidian egg corresponds to the " gray crescent" of the frog's egg. In both cases 

 it lies on the anterior side of the egg and in the vicinity of the dorsal lip of the blasto- 

 pore, but it is not positively known that chorda and neural plate are derived from it 

 in the frog, as is the case in the ascidian, though this seems probable. 



