Key Concepts:

Figure 1. Early patterning. (a) The oocyte in the ovary has an animal–vegetal polarity, with an animal pole (AP) and a vegetal pole (VP). There is yolk-poor cytoplasm in the animal half and yolk-rich cytoplasm in the vegetal half. The large oocyte nucleus, called the germinal vesicle (GV) is located in the animal half, and molecules, important for patterning the embryo, are localised in the vegetal cortex, near the surface of the vegetal half. The molecules include VegT RNA (purple v's) and dorsal determinants (green d's). (b) With oocyte maturation, the oocyte becomes ready for fertilisation. VegT RNA is released from the vegetal cortex and is translated into VegT protein (red v's). The egg reaches metaphase II of meiosis, and the small metaphase spindle is located at the animal pole (AP). The sperm enters the animal half. (c) Following fertilisation, the cortex rotates relative to the cytoplasm, shifting the dorsal determinants to one side. That side becomes the dorsal side (D), and the opposite side, where the sperm entered, becomes the ventral side (V). (d) The regions of the egg after cortical rotation have different fates in the embryo. The dorsal determinants are indicated by green dots, and there are three prospective germ layers: ectoderm, mesoderm and endoderm.

Figure 2. Cleavage. (a) Four cells result from two vertical divisions. (b) A horizontal division produces eight cells. (c) The 32-cell embryo has four tiers of eight cells. Experiments show that the eight cells along the dorsal side have dorsal information (green dots), with the highest concentration in the third tier. (d) Continued divisions produce the blastula. (e) The blastula has an internal cavity, the blastocoel, and the future mesoderm forms an internal ring. The future anterior (a) and posterior (p) polarity of the prospective notochord (red) is initially opposite to the future anterior and posterior polarity of the prospective central nervous system (CNS) (green). An, animal; Vg, vegetal; D, dorsal; V, ventral.

Figure 3. Fate maps and gastrulation. (a) At the beginning of gastrulation, the dorsal lip (dl) appears on the dorsal side, below the equator, and the areas of different fate can be marked on the surface relative to the dorsal lip. The region animal to the dorsal lip is the Spemann organiser (SO). Future lens of eye (white circle). Future mouth ectoderm (white triangle). Endodermal contribution to the mouth (M). (b) This picture shows the arrangement of fated tissues when the embryo in (a) is cut in the plane of the page. The prospective mouth ectoderm is at the top in both pictures. The invagination at the dorsal lip is due to a change in shape of some surface cells, and these cells are called bottle cells. ‘a’=anterior and ‘p’=posterior. (c) As the endoderm (yellow and tan) rolls inwards, the prospective epidermis (blue) and central nervous system (green) expand to cover the outside of the embryo. (d) In a section of the embryo in (c), cells migrate along the wall of the blastocoel (Bl), reducing its size. A new cavity, the archenteron (Ar) arises, with bottle cells at the leading tip. (e) At the end of gastrulation, all of the endoderm is internal, and the blastopore (B) closes at the posterior end. The anus will form near the closed blastopore. (f) In a section of the embryo in (e), the blastocoel (Bl) is almost obliterated, and the archenteron is greatly expanded. The anterior of the archenteron in particular expands due to a change in shape of the bottle cells. A mouth will form by perforating the epidermis and the endoderm, thus connecting the archenteron to the outside. The embryo in (e) and (f) elongates in an anterior–posterior direction as indicated by the large arrow. (g) After gastrulation, the tissues (green) developing into the brain and spinal cord form a tube and move internally. This leaves only epidermis (blue) on the surface. Compare (a), (c), (e) and (g). (h) A longitudinal section of the larva shows the major tissues. Compare this picture with (b), (d) and (f). (i) A cross-section shows the three tubes and the dorsal axial structures. Archenteron (A).

Figure 4. Gastrulation movements. (a) Gastrulation consists of complicated movements, indicated by the arrows. They include epiboly, vegetal rotation (VR), migration (Migr) and convergent extension (CE). CE will be followed in the succeeding drawings. (b) The fate map of the internal mesoderm ring has a dorsal (D)–ventral (V) polarity coinciding with the ectoderm and an anterior–posterior polarity opposite to the ectoderm. The notochord (No). (red) is the focus in (c)–(e). (c) This view of the mesodermal ring is rotated 90° counter-clockwise relative to that in (b), so that the notochord (red) is centred. The prospective anterior (A) is at the bottom, with the prospective posterior (P) at the top. There are two fields of prospective heart near the anterior edge. The arrows indicate the turning in and through the middle of the mesodermal ring. (d) After the notochord and mesoderm have turned in, lateral cells converge towards the midline (arrows), causing the prospective notochord to extend in an AP direction (double headed arrow). (e) As a result of CE, the notochord is elongated greatly towards the anterior end. The two heart fields will move together at the ventral midline and fuse.