Splayleg is a condition in which the hindlimbs and sometimes the forelimbs of a newborn pig temporarily are unable to support the body. If affected pigs are nursed through the period of high risk just after farrowing, they may compensate by enhanced synthesis of muscle proteins and then may grow normally. Slippery floors make the splayleg problem worse. Affected pigs may be helped with a loose coupling between the hindlimbs to prevent the limbs splaying outwards. In poultry, embryology is different to that in mammals because the developing embryo is flattened into a disk on top of the yolk.
Cleavage occurs as the zygote is passing along the oviduct before the egg is laid.
The cytoplasm is concentrated in the blastodisc at the opposite pole to the yolk mass. The roles of cells from various regions of the epiblast and hypoblast in the developing embryo may be determined experimentally by marking the cells with nontoxic dyes. Gastrulation is relatively simple in oligolecithal eggs, but in the flat blastodisc of poultry eggs gastrulation is more complex and the cellular intrusion characteristic of gastrulation occurs along a slit called the primitive groove . The primitive streak develops after the egg is laid and by about 16 hours of incubation is the most conspicuous feature of the developing embryo. The primitive streak indicates the location of the future vertebral axis.