AN ULTRASTRUCTURAL ANALYSIS OF CYTOPLASMIC LOCALIZATION IN CHAETOPTERUS PERGAMENTACEUS

¹ Department of Zoology, Howard University, Washington, D. C. 20059, and Marine Biological Laboratory, Woods Hole, MA 02543

During early development of Chaetopterus pergamentaceus, cytoplasmic components visible by electron microscopy became segregated. The hyaloplasm, or spongy layer, composed of granular bodies surrounded by a matrix of endoplasmic reticulum and microtubules was segregated towards (1) the upper pole of the unfertilized egg, (2) the interior of the cleaving egg, (3) the superficial regions of the 16-cell stage, and (4) the ectoderm of the trochophore larva. The ectoplasm, characterized by cortical granules, surrounded the unfertilized and cleaving egg endoplasm. It was restricted to the apical end of cells at the 16-cell stage, and to the ectoderm of the trochophore larva. The endoplasm, characterized by yolk and lipid, (1) was found throughout the unfertilized egg except at the upper pole, (2) was restricted to a ring just below the cortex of the cleaving egg, (3) moved to the basal ends of blastomeres by the 16-cell stage, and (4) was limited to the endoderm of the trochophore. Granular bodies were found throughout the embryo. Mitochondria, found throughout the cytoplasm by first cleavage, were absent from the hyaloplasm of the unfertilized egg. The cytoplasm of the polar lobe was similar to that of the surrounding endoplasm. The localization of morphogenetic determinants was thus accompanied by cytoplasmic organelle localization in Chaetopterus. The results are interpreted in terms of localization mechanisms involving the embryo cortex and ground substances.