Jae Do Yang, et al. abdominis due to early development before hernia resolu- tion. According to Munger and Munger,2 all developmental events involving nerves, vessels and muscles in the anterior abdominal wall are linked to a process of differentiation of the epidermis. However, in the present study, the rectus ab- dominis and its sheath commenced development prior to skin differentiation. Muscle fibers in the anterior abdominal wall muscles originate from somites,19 whereas the associated connective tissues originate from the lateral plate because myogenic cells migrate laterally beyond the lateral somitic frontier.20 Such muscles have recently been termed abaxial muscles, rather than the classical name hypaxial muscles.21,22 There- fore, the rectus sheath also seems to originate from the lat- eral plate. However, the rectus sheath is unlikely responsi- ble for the pattern of formation of the muscle, but is rather more likely to form later under mechanical influence of the three-layered lateral muscles. Notably, at the earliest stage, the superior part of the rectus abdominis was at the same level as the brachial plexus. The rectus abdominis suddenly stopped upward extension at the margin of the inferiorly developing pectoralis major. Thus, the layer or position of the rectus abdominis seemed to correspond to that of the pectoralis at the superficial side of the anterior chest wall. The external intercostal was outside of the competitive rela- tionship for the common territory between the pectoralis and rectus abdominis. According to Evans, et al.,23 in con- trast to non-migratory lateral abdominal muscles, the rectus muscle anlage migrates along the body wall, similar to the pectoralis muscle anlage. Using a chick-quail chimera system, Liem and Aoyama22 recently demonstrated that an ectopic limb experimentally induced on the superficial side of the lower chest wall often interfered with development of the rectus abdominis. The pectoralis-like muscle was observed to additionally appear in the original abdominal wall instead of the rectus abdomi- nis, even though the chick rectus abdominis is derived from only 1 or 2 segmental anlagen. The rectus thoracis, if pres- ent in humans (e.g., Mehta, et al.24), is likely to correspond to part of the pectoralis major such as the sternalis. The sternalis, a well-known variant similar to the supracostal part of the rectus abdominis, is supplied by the pectoral nerve(s).25,26 O’Neill and Folan-Curran27 reported on the sternalis with muscle fibers that directly communicated with the pectoralis, and various patterns were reviewed by Jelev, et al.28 The sternalis was also reported in combination with the absence of the sternocostal part of the pectoralis.29-31 Moreover, similar to the rectus abdominis, the sternalis may sometimes be innervated by the intercostal nerve.8-12 Since the rectus-like or sternalis-like muscle appears in hoxb4 mu- tant mice along with failure of the bilateral sternal anlagen (Manley, et al.15; see above), the common variant is likely to result from a slight discrepancy in timing between chest wall closure and the inferomedially developing pectoralis major. We concluded that in fetal development as well as in evolution, the pectoralis interferes with supracostal expan- sion of the rectus abdominis. Finally we discussed the medial and lateral perforator rows of the inferior epigastric artery that is of great interest to surgeons.13 Rozen, et al.13 considered that, because there are two muscle heads very early on in its development, each head draws its own blood supply to provide “a perforator row” for each head. However, we have provided evidence against this possibility (see the first paragraph of the Dis- cussion). In short, if we use the terms of “origin and inser- tion” for anatomical description of muscles, the rectus ab- dominis seemed to develop from its insertion to its origin, not from the origin to the insertion. In three of four of the present specimens at 9 weeks, in which the rectus abdomi- nis grew rapidly at the level below the umbilicus, the inferi- or epigastric artery divided into two major branches at the level above the umbilicus. Although the present specimens were very limited in number, this variation appears not to be so different from that known in adults.13 The two-row ar- rangement of the perforators seems to depend on a fact that there are two major arterial branches running longitudinally in fetuses. The number of branches may depend on fetal muscle thickness and width, not on the hypothetical two columnar anlagen of the rectus abdominis. ACKNOWLEDGEMENTS This study was supported by a grant (0620220-1) from the National R & D Program for Cancer Control, Ministry of Health & Welfare, Republic of Korea. REFERENCES 1. Williams PL. Gray’s Anatomy. 38th ed. Edinburgh: Churchill Liv- ingstone; 1995. p.825-9. 2. Munger GT, Munger BL. Differentiation of the anterior body wall and truncal epidermis and associated co-migration of cutaneous nerves and mesenchyme. Anat Rec 1991;231:261-74. 1034 Yonsei Med J http://www.eymj.org Volume 53 Number 5 September 2012