ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 13 



Van Beneden and Julin (1884, pp. 4 and 5) call attention to the fact that 



Corella parallelogrannna produces two kinds of eggs, one yellow the other gray in 

 color. Both kinds of eggs develop normally and in the same manner and give rise 

 to larva', the endoderm cells of which are colored yellow or gray. Clavellina risso- 

 ana also produces two differently colored kinds of eggs, one pure rose the other 

 yellowish in tint. All eggs produced by the same individual have the same color. 



In the species of Clavellina studied 1>\ Seeliger (1885) the color of the proto- 

 plasm surrounding the nuclei of the cleavage cells is yellow, as an inspection of his 

 figures of the living eggs shows (v. his plate I). 



These cases, taken in conjunction with my observations on the eggs of Cynthia, 

 Ciona, Molgula and Bollema, show that the eggs of ascidians are frequently 

 colored; these colors are usually found in the yolk, and in the later development 

 pass into the endoderm cells. In Cynthia the peripheral layer is also colored, and 

 this fact leads me to hope that some other ascidian may be found in which still 

 other portions of the ooplasm may be differentially colored. 



II. MATURATION AND FERTILIZATION. 



These processes are so intimately associated in the ascidian egg that it is diffi- 

 cult and perhaps inadvisable to treat them entirely separately. As in so many 

 other eggs the entrance of the spermatozoon furnishes some stimulus to the egg 

 which leads to the completion of the maturation divisions. Without this stimulus 

 the egg may remain in the stage of the metaphase of the first polar spindle for 

 hours or even days. 



A. Maturation. 



1. Disappearance of Nuclear Membrane. 



The first steps in the formation of the polar spindle take .place before the 

 entrance of the spermatozoon. Almost as soon as the egg is laid, and sometimes 

 even before this, the wall of the germinal vesicle dissolves and the clear protoplasm 

 contained within the germinal vesicle moves up to the animal pole of the egg where 

 it may spread out into a cap or peripheral layer (Ciona), or may form merely a 

 somewhat flattened disk (Cynthia). As soon as the nuclear membrane has dis- 

 solved the chromosomes, nucleolus and a granular mass from which the spindle 

 fibres are formed gather together into the center of this area of nuclear proto- 

 plasm (figs. 62, 63, 77, 78) ; since the chromosomes lay at the periphery of the ger- 

 minal vesicle before its membrane dissolved, this involves a considerable movement 

 on the part of these various constituents. No distinct linin network is visible 

 throughout the germinal vesicle, either before or after its membrane dissolves, and 

 the drawing together of these scattered elements into a central mass must be due 

 to something other than the contraction of the threads of such a network. 



The chromosomes, when drawn together into a central mass, are connected by 

 a faintly staining, finely granular substance, which is much denser than the sur- 

 rounding nuclear protoplasm. In the further development of the polar spindle this 

 mass gives rise to the spindle fibres, and from this fact, as well as from its staining 

 reactions, it may be identified with linin (figs. 62, 77). 



