Hybridization of Echinoids. 27 



in other plates, would appear in the polar view as short rods because 

 of their oblique position. With this interpretation the number of 

 chromosomes in these plates is either 37 or 38. 



The final proof of these specific numbers of chromosomes in Cidaris 

 was gained by a study of the eggs activated artificially. Unfor- 

 tunately, in the material that I have studied I have been unable 

 to find any amphiasters. The figures that show well are all of mon- 

 aster plates. The most favorable of these is shown in figure 28. 

 The chromosomes have all divided longitudinally on the monaster. 

 Nineteen such divided chromosomes were present in one section, 

 the remaining section of the egg containing no additional chromo- 

 somes. In this plate I am unable to designate the heterochromosomes 

 with any degree of certainty; it may be one of either of the two 

 rounded elements in the lower right-hand corner of the plate. The 

 U- or V-shaped chromosomes may, however, be seen in most plates, 

 as in figures 26 and 27a. Figure 26 was drawn from two sections. 

 With the exception of two chromosomes, all were in one section. 

 The total number of chromosomes here also was 19. In all sections in 

 which the count could be made with certainty, 19 was the number of 

 chromosomes present. Such figures as 27a and 276 are inconclusive 

 as to numerical count, because of the evident fragmentation of the 

 chromosomes in sectioning. 



Chromosomes in CroARis Eggs Cross-Actfvated by Lytechinus Sperm. 



Cidaris 9 X Lytechinus cf . 

 Figures 12 to 17. 



A comparison of figures 6 to 11 with figures 12 to 17 shows a 

 general similarity in the character of the anaphase plates, but it also 

 reveals some rather striking differences. As a rule, division seems 

 to be rather regular, but even in the most regular figures some evi- 

 dence of lagging in the separation of the chromosomes is evident 

 (figs. 12 to 15). So far as the morphology of the chromosome 

 groups is concerned, I have found that the eggs may be divided into 

 two groups: one group in which each anaphase plate contains two 

 of the V-shaped or broader chromosomes, and one group in which 

 each anaphase plate contains three of these heterochromosomes. 



In the preceding section evidence is presented that the Cidaris egg 

 contains one of these heterochromosomes. It is therefore evident 

 that the Lytechinus sperms are dimorphic, carrying either one or two 

 V-shaped heterochromosomes. Figures 12, a and b, show two chro- 

 mosomes of double width; figures 13, a and b, show three such hetero- 

 chromosomes in each plate; figures 14, a and b, show three; figures 

 15, a and 6, show three such chromosomes in the lower half of the figure 

 and three in the upper half. 



