Chromosome number of sea urchin andromerogones during early development

The chromosome number of the andromerogones obtained by fertilizing non-nuc leate egg fragments of Hemicentrotus pulcherrimus was examined by an air-dr ying method during early development. The non-nucleate egg fragments were p repared by centrifuging unfertilized eggs in a stepwise saccharose density gradient with a purity of 99.9%, comprising from about one-tenth to one-fou rth of the total egg volume. The andromerogones cleaved, hatched and then a ctively swam. Though their development tended to be slower compared with th at of control whole embryos after hatching, most of them developed into lar vae and the remainder developed as permanent blastulae. While the larvae ha d differentiated gut and pigment cells, they also had various skeleton type s which varied from almost normal to irregular. The chromosome preparations were made from blastomeres dissociated from many andromerogones. The rate of cells having a haploid chromosome number of 21 was 73% at the two cell s tage, 84% at the 8 cell stage, 75% at the morula stage, 76% at the hatching blastula stage and 87% at the swimming blastula stage, while that of cells having a diploid number of 42 was 2% if averaged out for total cells exami ned. A total of 73-87% of the andromerogones was found to develop with a ha ploid number of chromosomes.