Postfertilization Changes in the Horseshoe Crablimulus Polyphemusl

Postfertilization changes occurring during early development in the American horseshoe crab, Limulus polyphemus, have not been completely clarified in the literature. To help alleviate this problem, the present study examines the first four of twenty-one developmental stages as outlined for the Japanese horseshoe crab, Tachypleus tridentatus, by Sekiguchi (1973) and for Limulus by Brown and Clapper (1981). These four stages involve cleavage (Stages 1-3) and early blastulation (Stage 4). Cleavage consists of two parts: 1) intralecithal cleavage and 2) total cleavage. The first part (Stages 1 and 2) involves nine postfertilization events which are emphasized in this study and include the cortical reaction and three intermittant granulations. During the first two granulations, granules appear simultaneously over the entire surface of the fertilized egg. The third granulation differs from these in that granules appear locally on the embryo and gradually progress over the surface. This granulation is also characterized by a contraction wave passing over the surface. The first part of cleavage (intralecithal cleavage) ends with the appearance of nuclei near the surface (Stage 2). The second part of cleavage (total cleavage, Stage 3) then proceeds but is somewhat modified from a standard holoblastic cleavage since the planes of each division are not completed.