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Fig. 2. Overview of Patella early development: 3D induction and AP axis specification. Unless mentioned explicitly, all views are taken from the vegetal pole. D quadrant (posterior) is to the bottom, so the left side of the embryo is to the right. Drawings are freely adapted from van den Biggelaar (van den Biggelaar, 1977 ), except A, which is adapted from Wilson (Wilson, 1904), and G. (A) 4-cell stage (30 minutes after first cleavage). The four cells are of equal size, and the progeny of each of them is called a quadrant. (B) 32-cell stage (2 hours after first cleavage (h.p.f.c.)), displaying a four-fold symmetry. (C) Early 32-cell stage (2.5 h.p.f.c.), meridional cross section, before 3D determination. Animal is to the top. Two of the four vegetal cells (macromeres) can be seen (3M). During this period, the four macromeres invade the blastocoel, and compete for contacting the animal micromeres that constitute the blastocoel roof. (D) Mid 32-cell stage (3 h.p.f.c.), same section as in C: one of the four macromeres contacts the blastocoel roof, and as a consequence, takes a 3D fate. (E) 60-cell stage (3.5 to 4 h.p.f.c.). 3D looks smaller, since most of its mass is internalised. 3a, 3b, 3c and 3d are in metaphase. Together with 3D, these latter cells’ cleavage adumbrates bilateral symmetry. (F) 64-cell stage (4 h.p.f.c.): 3c and 3d have yielded smaller vegetal cells (3c2 and 3d2) and larger animal cells (3c1 and 3d1), whereas 3a and 3b have divided in an opposite fashion, budding off a larger cell towards the vegetal pole (3a2 and 3b2. Note that 3a1 and 3b1 are not visible on this drawing). 3D is the last cell to divide before completion of the 6th cleavage, and yields 4d, or M, the mesentoblast, that will contribute to most of the adult mesoderm. (G) 2 hours after the 64-cell stage (6 h.p.f.c.). The segregation of the prospective germ layers is now achieved. The endodermal vegetal plate (yellow) is made from all descendants of 3A, 3B, 3C and 4D. 4d, the mesentoblast divides in a bilateral fashion, yielding Ml and Mr, the paired stem cells giving rise to the mesodermal germ bands (red). 3a and 3b derivatives also contribute to mesoderm (light orange) (Dictus and Damen, 1997). Cells coloured in dark blue are of ectodermal fate, and delineate the posterior edge of the blastopore. The pink blastomeres are the three stomodaeum founder cells, and are thus called stomatoblasts, as in Nereis (Wilson, 1892). During gastrulation, the vegetal plate is internalised by epiboly: the rim of ectoderm migrating over and enclosing the vegetal plate is equivalent to a blastopore edge which, in the case of gastropods, gives birth to the mouth. The dark blue cells constitute a useful landmark: they are at the edge of the vegetal plate, and will contribute to the ectoderm at the base of the larval mouth, and thus define the posterior edge of the blastopore.
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