## 3 Results and Discussion
\begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline  & \multicolumn{3}{c}{**Kuts**} & \multicolumn{3}{c}{**Kieku**} & \multicolumn{3}{c}{**Litesg**} & \multicolumn{3}{c}{**Madona**} \\  & **H/HN/BCE** & & \multicolumn{3}{c}{**H/D/BWE**} & \multicolumn{3}{c}{**H/LS/CCE**} & \multicolumn{3}{c}{**H/HN/CCE**} \\ \cline{2-13}  & **ES** & **SMB** & **WMC** & **ES** & **SMB** & **WMC** & **ES** & **SMB** & **WMC** & **ES** & **SMB** & **WMC** \\ \hline Average size, \(\upmu\)m & 362.6 \({}^{\text{a}}\) & 53.2 \({}^{\text{b}}\) & 0.67 \({}^{\text{c}}\) & 366.4 \({}^{\text{a}}\) & 59.9 \({}^{\text{a}}\) & 1.48 \({}^{\text{a}}\) & 356.2 \({}^{\text{a}}\) & 20.0 \({}^{\text{c}}\) & 1.41 \({}^{\text{a}}\) & 362.4 \({}^{\text{a}}\) & 21.4 \({}^{\text{c}}\) & 0.91 \({}^{\text{b}}\) \\ STDEV & 22.9 & 5.3 & 0.22 & 17.7 & 8.5 & 0.14 & 19.6 & 4.2 & 0.18 & 11.5 & 5.3 & 0.10 \\ \hline \end{tabular} Note: Values are means \(\pm\) SD values of \(n=50\). Means within the same attribute with different superscript letters (\({}^{\text{a}}\), \({}^{\text{b}}\), \({}^{\text{c}}\)) are significantly different at \(p\leq 0.05\). SD—standard deviation; ES—size of eggshell; SMB—size of eggshell membrane; WMC—width of surface microcracks.  
\end{table} 
Table 2: Mean size values of eggshell and membrane thickness and surface microcracks captured by scanning electron microscopy at 0 days of eggs’ storage produced by laying hens of various origins and housing systems. Figure 3: Elemental mapping analysis (**A**) and energy dispersive spectroscopy (**B**) of the eggshell derived from eggs of DeKalb laying hens aged 42 weeks and housed under cage-free conditions. Note: S—sulfur; P—phosphorous; Mg—magnesium; Ca—calcium; C——carbon; O——oxygen. Figure 2: The external surface of the eggshell of laying hens captured by scanning electron microscope showing longitudinal hairline and spider-like cracks (indicated by the yellow L letter with width values). Note: Scanning electron micrographs were taken at the initial stage of storage (0 days). (**A**)— eggs from cage-free housed laying hens (H/HN/BCE); (**B**)—eggs from cage-free housed laying hens (H/D/BWE); (**C**)—eggs from enriched cage-housed laying hens (H/LS/CCE); (**D**)—eggs from enriched cage-housed laying hens (H/HN/CCE). During the elemental mapping analysis, the presence of P was revealed (on average, 1.50% per weight), mainly in the form of calcium phosphate (Ca\({}_{3}\)(PO\({}_{4}\))\({}_{2}\)), as reported by [PERSON] et al. [39]. This statement can be reinforced by the same uniform distribution pattern of Ca and O elements as P throughout mammillary and palisade layers of the eggshell. It is worth noting that during the EDS mapping analysis, the presence of P with high signal intensity was also observed in the outer part of the eggshell. Meanwhile, the same intense deposition of Mg near the inner and outer surfaces of the eggshell (on average, 0.92% per weight) was revealed. The presence of Mg in the form of magnesium carbonate (MgCO\({}_{3}\)) salt was reported by [PERSON] and [PERSON] [40]. This observation can be supported by the porous nature of the eggshell and the outstanding solubility of magnesium salts and phosphates rather than calcium salts by the alcohol mixtures and aqueous acid [41]. The levels of O, C, and Ca elements were the highest among all the elements, corresponding to 48.3, 30.1, and 19.8% distribution by weight, respectively. The availability of C (carbon) as the principal element observed along with Ca and O is explained by the double-sided adhesive carbon disc applied for the fixation of eggshell specimens. Previous studies with an EDS mapping analysis of eggshell interior revealed the availability of calcium carbonate (CaCO\({}_{3}\)), which makes up 97% of the total amount of elements [42].