SIX FIGURES
Recently, workers using the electron microscope have obtained visual evidence of physical changes which occur in nuclei during sperniiogenesis. Grass6 et al. ('56) first presented evidence that the chromatin of a pulmonate snail, UeEix, takes the form of highly oriented fibrils in the spermatid. Rebhun ( '57) also working on pulmonate snails, reported similar events, though his interpretation of nuclear structures as sheets is in disagreement with that of Grass6 e t al. Homologous nuclear structures have also been described in orthopteran spermatids by a number of workers (Gibbons and Bradfield, '57a; Yasuzumi and Hiroshi, '57; Dass and Ris, '58 ; Sjostrand and Afzelius, '57) ; in these cases the chromatin differentiates to form a number of closely packed tubules in the late spermatid.
I n the following work nuclear cveiils during spermiogenesis in a prosobranchiate gastropod have been followed with the electron microscope. A more exact description of certain aspects of the differentiation of the nuclear structures has been made than in the above mentioned worlrs. I n addition it has been found that in the late spermatid the chromatin takes on a unique lamellar arrangement.