**1. Introduction**

In recent decades, the need to increase hatchery efficacy is increasing with demand for quality chicks. Therefore, during incubation, new techniques which are associated with embryo management are increasing with this demand. A possible reason is that the newly developed broiler genotypes have diverged considerably compared to traditional genotypes in terms of the biological, physiological, and biochemical requirements [1]. Thus, manipulation of different incubation conditions to meet the requirements of modern broiler genotypes is under continuous investigation. Incubation conditions have a significant effect on hatchability, chick quality and post-hatch performance in chicken. In addition, while the first quarter of incubation is critical to chicken embryogenesis [2], the last quarter is very significant to hatch and post hatch performance [3].

Incubation temperature is the most significant incubation condition [4] and there has been an increase in studies regarding thermal manipulation (TM). TM (i.e., increasing or lowering incubation temperature) and broiler chicken embryogenesis has been deeply studied by Collin A, Tzschentke B, Piestun Y, Yahav S, and Halevy O, and the technique enhances chick quality through improved body weight gain, increased expression of genes in the breast muscle, and thermotolerance. Earlier studies laid the foundation for implementation of TM between different days of embryogenesis in addition to key factors such as duration and strength of TM to enhance the chick's ability to cope with environmental challenges of cold and heat stress during post-hatch period [5–10].

In an earlier study, Yahav et al. [11] reported that TM at 39.5°C for 3 hours (h)/day (d) from embryonic age (E) E11–E16 improves the chicks' thermotolerance acquisition. A similar effect was confirmed by [12–16] with TM at 39.5°C for 12 h/d from E7 to E16. Recent studies have also confirmed a long-lasting effect on thermotolerance acquisition in chicks at hatch or 1-day-old chicks, for instance, Piestun et al. [17] and David et al. [18] both with TM at 39.5°C for 12 h/d from E7 to E16, Al-Zghoul et al. [19] with TM at 38.5, 39, 39.5 and 40°C for 18 h/d from E12 to E18 and Al-Zghoul et al. [20] with TM at 39°C for 18 h/d from E10 to E18.

The effect of TM on hatchability in several studies has been reported to differ extensively, with hatchability after TM being higher [11, 21, 22], reduced [9, 12, 13, 23, 24], and not affected [25, 26]. Also, studies from different researchers have shown contradicting results on chick quality parameters, and hatch or chick weight after TM exposure was increased [22, 23, 27], decreased [28], and not affected [21, 25, 29].

Production of optimal quality chicks depends on controlling incubation conditions and understanding the insights into the complex interaction among them. Although various studies have reported the benefits of TM, results are still contradicting and depend on timing, duration, and level of TM. With this background, we systematically reviewed the recent literature regarding the effect of thermal manipulation on embryonic development, hatchability, and hatching quality of broiler chicks.
