CHROMOSOMES IN CROSS-FERTILIZED ECHINOID EGGS. 1 29 



clei, formation of the amphiaster, etc., is then approximately the 



same as in Toxopneitstes eggs fertiUzed with Toxopneustcs sperm. 



The chromosomes as seen in a polar view of the equatorial 



nlate of '^:^^:^P!!!'!f^ eggs are shown in Figs, i and 2. 

 piaie 01 j^^op^igustes ? ^"^ 



They are seen to have the appearance of rather long, slender, 



and somewhat bent rods. By comparing these two figures it 



may be seen that variations in the form of the chromosomes, 



which are correlated with slight differences in the ages of the 



plates, are evident. 



A corresponding view of the chromosomes in a section of a 



^ egg IS shown m t\g. 3. 



Some differences are apparent, but in general the size, form, 

 etc., of the chromosomes in this plate are so like those of the 

 Toxopneustes egg that one need scarcely venture to hope to be 

 able to identify the chromosomes of maternal and paternal origin 

 in the cross-fertilized eggs. 



An examination of sections such as are illustrated in Figs. 4 

 and 5 convinces me that we have here a mixture of the two 

 sorts, but I find myself unwilling or perhaps unable to distin- 

 guish the chromosomes of either origin. 

 "^ Some interesting variations from the normal were found in one 



series of -^r^^P'^^^^^ i" ^^'h^^^^ "^°^'^ ^^^^" °"" spermato- 

 Toxopneustes ¥ 



zo6n had entered the tgg. Two different results are shown in 

 Figs. 9-12 and Text Fig. i. 



In one case the extra sperm-nucleus is seen moving toward 

 the segmentation nucleus. Its aster has divided while the cen- 

 trosome of the future cleavage amphiaster is still single (Fig. 9). 

 Later the second sperm nucleus seems about to fuse with the 

 segmentation nucleus while the centers of the regular cleavage 

 amphiaster have separated (Fig. 10). In some cases fusion 

 between the two nuclei takes place; in others (Fig. n), the 

 fibers from one of the sperm asters enter the nucleus and the 

 chromosomes become differentiated in the network before the 

 cleavage asters have well separated. 



In the other case (Fig. 12 and Text Fig. i), chromosomal 



