Fine structure of the mitotic cycle of unfertilized sea urchin eggs activated by ammoniacal sea water.

Unfertilized sea urchin eggs enter a mitotic chromosome cycle after treatment with sea water containing ammonia. Centrioles cannot be found but microtubules are formed in the later stages of the cycle. The microtubules are displayed in an astral arrangement centered on clusters of osmiophilic bodies. In early stages, distinct kinetochores on the condensed chromosomes show no attachments to microtubules. Later, a few microtubules may be attached to the kinetochores. The chromosomes and microtubules are contained in a "clear zone", a large compact accumulation of membranes which displaces yolk particles and mitochondria, but not ribosomes, from that region of the cell. No bipolar spindle is formed.