FERTILIZATION AND CLEAVAGE IO3 



measurements of metaphase plates of first cleavage stained with aceto- 

 carmine, a medium-sized, spherical chromosome has a diameter of 

 ca. 0.8 [I, giving a volume of ca. 0.268 [j.'. All the 38 chromosomes 

 would have a volume of ca. 10.2 a^, or 1/20,000 the volume of the egg. 

 The sperm head, containing the haploid number of chromosomes, 

 has a volume of ca. 3.4 (x^, as calculated from the data (length 3.25 {x, 

 thickness 2 [x) of E. B. H. (see under Sperm, Chapter 12a). The volume 

 of desoxyribose nucleic acid in the Arbacia sperm head is 1.3 to 1.4 ]x^ 

 calculated by Mazia (personal communication 1955). For Drosophila, 

 Sturtevant (personal communication 1950) calculated that the diploid 

 chromosomes have a volume of ca. i.o [j.^, and the sperm head (hap- 

 loid) about one quarter of this. Other figures for Drosophila from which 

 similar volumes can be calculated have been given by Muller (1929) 

 and Gowen and Gay (1933). 



i. Visibility of the Cleavage Figure 



In the living egg of Arbacia, asters are plainly visible, but spindles and 

 chromosomes cannot be observed with the usual light microscope even 

 with the best apochromatic objectives and compensating oculars. When 

 no granules are present, as in the clear quarters of the centrifuged egg, 

 the asters cannot be seen. In photographs taken with ultraviolet light, 

 the chromatin material and chromosomes of the living egg appear as 

 they do in fixed and stained sections (Harvey and Lavin, 1944). With 

 infrared light, photographs of the living egg show the configuration 

 of the mitotic figure in a striking manner as a brilliant white area 

 against a dark granular cytoplasmic background (Harvey and Lavin, 

 1951 b). The spindle of Arbacia has not been studied with a polarizing 

 microscope, but the mitotic figures of other sea urchin eggs show a 

 beautiful birefringence (Swann, 1951a, b; Innoue and Dan, 1951; 

 see the fine photographs in these papers and Hughes', 1952, book, 

 Plate XIII). 



The structure of the mitotic figure is best studied in sectioned 

 material. The best fixative is probably Bouin, although the egg shrinks 

 from 74 to 50 [i.; the most satisfactory stain is Heidenhain's haema- 

 toxylin. Very good total mounts can be made with aceto-oarmine and 

 acetic orcein. A photograph of a very thin section of a spindle taken 

 with the electron microscope, has been made by Geren and McCulloch 



(1950- 



Fading out of cleavage figure or furrow (reversible) is caused by 

 Colchicine, Nebel 1937; Nebel and Ruttle, 1938; Beams and Evans, 

 1940. 



