Fig. 3

Early development of M. japonica. A Fertilized egg forming the first polar body. B Formation of the second polar body. C First cleavage. D Second cleavage furrow. E The 4-cell stage, polar view. F The 8-cell stage, lateral view. G The 32-cell stage, lateral view. H Morula stage. I Prehatching stage, rotating in the egg chorion with an apical tuft and cilia. J 72-hour-old embryo with the posterior end fading. K 84-hour-old larva hatching from the egg chorion. L 84-hour-old larva with a prominent apical tuft, anteroposteriorly becoming elongated. The posterior region appeared lighter colored. M 4-day-old larva showing a conspicuous posterior invagination. N 5-day-old larva with a posterior cirrus. O 7-day-old larva showing the foregut. P 14-day-old larva showing the mouth opening and anteroposteriorly elongated foregut. Q 21-day-old larva with the intestine no longer filled with yolk granules. In this stage, posterior invagination resulted in a distinct sucker-like structure in the ventroposterior end. The arrowheads point to the posterior invagination. Scales: 50 μm (A–H); 80 μm (I–Q). Abbreviations: ap, apical tuft; fg, foregut; mo, mouth; pb, polar body; pc, posterior cirrus