Balanoglossus-prelarval development

The prelarval development is similar to that of Branchiostoma. The zygote, undergoes cleavage which is holoblastic, almost equal and of the radial type. It result into a sphere of blastomeres, the morula. The morula undergoes into blastomeres formation and form of a single-layered,hollow and spherical embryo,the blastula or coeloblastula. Its cavity called blastocoel. Blastula result in about 6-15 hours after fertilization.Within 12 to 24 hours,an invagination starts in the blastula which deepens to form to form the archenteron that opens to outside through a blastopore.Soon the blastopore closes and the embryo,now called gastrula.The tip of archenteron is pinched off as a coelomic vesicle called the protocoel.Thus origin of coelom isenterocoelic.The remaning portion makes gut.The protocoel becomes trigular in shape.The hydropore, towards the dorsal side of the embryo.The protocoel and hydropore are the proboscis coelom and proboscis pore.The collar and trunk coelom arise as solid invagination of the hindgut,independent of the formation of protocoel.