sentences:  During early development, the sea urchin embryo undergoes 10 cycles of cell division, resulting in a single epithelial layer enveloping the blastocoel.The embryo then begins gastrulation, a multipart process which dramatically rearranges its structure by invagination to produce the three germ layers, involving an epithelial-mesenchymal transition; primary mesenchyme cells move into the blastocoel and become mesoderm.It has been suggested that epithelial polarity together with planar cell polarity might be sufficient to drive gastrulation in Sea urchin.