ABSTRACT
The embryology of many sea star species is unusual in that it forms a wrinkled blastula rather than the typical smooth-walled blastula seen in most other animals. The process of wrinkled blastula formation was investigated in the sea star Patiriella exigua by scanning and transmission electron microscopy. Wrinkling occurs as early as 12hrs after fertilization with the wrinkles at its deepest and most complex at 15 to 16hrs after fertilization. Wrinkled blastulae have a highly contorted appearance due to the infolding of the epithelium. After the peak of the wrinkled stage, the invaginations become shallower and decrease in complexity to form a gastrula by 24hrs after fertilization. Prior to the appearance of wrinkles, both types of nutritive material are evenly distributed throughout cuboidal cells. Coincident with the wrinkling process, the cells begin to elongate and the two yolk types start to display apical and basal polarity, respectively.
