102 THE AMERICAN ARBACIA 



describes them as occurring in this form. They are mentioned as 

 occurring in Lytechinus by Wilson in his Atlas (1895, P- 27 and Fig. 

 XVII), but do not appear conspicuous or typical. 



h. Chromosomes 



The chromosomes of Arbacia punctulata are small and crowded and 

 difficult to count. The diploid number, in cleavage cells, is probably 38. 

 This number, 38, is also given, with drawings, by Matsui (1924) ; and 

 Morgan (1927, p. 627 footnote) says that 36-38 is recorded by E. B. 

 Wilson and students. E. B. Harvey (1940c) gives 32-38 as the diploid 

 number, and half that number for the parthenogenetic egg. Tennent 

 (1912b, p. 397) says "about 40" (this article was incorrectly attributed 

 to Jordan, 1912, in my tabulation, 1920, p. 12). 



In Arbacia lixula, the Naples species, Baltzer (19 10) gives 40 as the 

 number of chromosomes in cleavage cells. In Lytechinus ( Toxopneustes ) 

 variegatus which closely resembles Arbacia in cytological details, the 

 number is 36 or 38 (E. B. Wilson, 1895, Atlas, p. 23; 1901a, b; Ten- 

 nent, 1912b). The chromosome numbers of the other Echinoidea are 

 given in the tabulation of E. B. Harvey, 1920, p. 12-14. This list 

 (including a mistake) has been copied in Tabulae Biologicae, vol. IV, 

 1 08 ( 1 92 7) by Breslau-Harnish, and in vol. 1 8, p. 34 ( 1 939) by McClung. 

 Makino, 1951, has added four more references to this list, including 

 Arbacia punctulata (38 chromosomes) and Echinarachnius parma (52 chro- 

 mosomes) by Matsui, 1924; Clypeaster rosaceus (44 chromosomes) by 

 Gardiner, 1927; and Strongylocentrotus intermedius (50 chromosomes) by 

 Niiyama and Makino, 1947. To make the list complete there should 

 be added Cidaris tribuloides with 37, 38 diploid, 18, 19 haploid, 19 

 parthenogenetic eggs (Tennent, 1922); and Mespilia globulus with 38 

 diploid, 19 haploid (Tennent and Ito, 1941). It will be seen that most 

 sea urchins have 36-38 chromosomes, diploid, including Arbacia 

 punctulata. 



There are apparently no sex chromosomes in Arbacia; at least none 

 have been reported. But there are sex chromosomes in Lytechinus varie- 

 gatus and Tripneustes esculentus (Tennent, 1911b, 1912 a, b, c, 1922); in 

 Paracentrotus lividus and Psammechinus microtuberculatus (Baltzer, 191 3); 

 and in Cidaris tribuloides (Tennent, 1922). The digametic sex is the male 

 and not the female as was once thought. (Baltzer, 1909, 19 10, 19 13). 



The chromosomes of Arbacia are small and of different sizes and 

 shapes; some are spherical and some rod-like (Tennent, 1912 b; Matsui, 

 1924; E. B. Harvey, iq40c. Photograph 130 on Plate VIII). From 



