Development of the mesoblast 4d. (A) Developmental stages of Playtnereis dumerilli: the embryo undergoes spiral cleavage and develops into a free swimming trochophore larva within 24 hours post fertilization (hpf). A cluster of vasa-positive cells is detectable at the base of the mesodermal bands. One day later, the anlagen of the first three segments as well as the stomodaeum are detectable in the metatrochophore larva. The nectochaete larva (72 hpf) exhibits three parapodial segments with bristles. The four PGCs leave the MPGZ in young worms of 96 hpf. Blue: mesodermal bands, green: PGCs and MPGZ, grey: gut. (B) Formation of the primordial germ cells, same colors are used as in (A), lateral view, modified after ; figures 72 and 73. The animal pole, defined by the position of the polar bodies and later by the apical organ, is pointing to the top. The blastopore (asterisk) lies at the future ventral side. The MPGZ demarks the posterior pole, while the somatoblast lies on the dorsal side. Embryos are drawn as optical sections in lateral orientation with anterior to the top and dorsal to the right. Only the right half of the embryo is shown in detail, the corresponding cells on the left side are shown with dashed lines. During epibolic gastrulation, the micromeres overgrow the macromeres, the future endoderm. Around 4.5 hpf, the mesoblast 4d buds of from the macromere 3D at the ventral ridge of the closing blastopore (asterisk). The mesoblast divides bilaterally around 5.5 hpf, giving rise to the two primary mesoblasts 4d1 and 4d2. At 6.5 hpf, the right mesoblast buds off the small secondary mesoblast cell 4d12 towards the blastopore. A second small blastomere (4d22) arises simultaneously from the left mesoblast (dashed lines). At 7.5 hpf, the primary mesoblasts give rise to a second pair of secondary mesoblasts, 4d112 and 4d212. (C) Embryo at 10 hpf, exhibiting the four Vasa-positive secondary mesoblast cells at the posterior rim of the blastopore (dashed line). Ventral view on the blastopore, anterior is to the top. Scale bar = 50 μm.