THE AMERICAN ARBACIA 



Fig. 1 1 . Drawing of Arbacia punctulata spermatozoon. 



"there is an exceedingly minute opening in the point of the head", 

 through which "one gets the impression that the spermatozoon ehmi- 

 nates very small amounts of an extremely sticky substance". The head 

 of the sperm contains the haploid group of chromosomes which have 

 a volume, if they are the sole components of the sperm head, of about 

 3.4 [j,^. A diploid group of 38 chromosomes from a first cleavage cell in 

 a smear preparation stained with aceto-carmine has a volume of about 

 10.2 [Ji^ (average diameter of a spherical chromosome is about 0.8 (x), 

 a little more than double that of the sperm head. The expected double 

 relation is fairly close when one considers the uncertainty as to how 

 much of the material is pure chromatin both in the diploid plate of 

 chromosomes and in the haploid sperm head. The amount of DNA in 

 the Arbacia sperm is 0.9 x io~^ micrograms, calculated as 1.3 to 1.4 [x^ 

 according to Mazia (personal communication, 1955), or about one 

 third of the volume (or weight) of the head. Marshak and Marshak 

 (1953) give 7.9 X io~^ micrograms in one sperm. 



The middle piece of the spermatozoon of Arbacia punctulata contains 

 two large spheres, probably centrosomes, especially well seen with the 

 phase microscope. These bodies in the middle piece stain purple with 

 methyl green and blue with Janus green, indicating the presence of 



