. The chordates. Chordata. Reproduction 269 MC contiguous to the endoderm. The thick part is destined to form a seg- ment of body-muscle and is therefore called the myotome (Fig. 223, M). The outer layer, being, in con- junction with the ectoderm, the body-wall of the embryo, is called the somatic or parietal layer. The inner layer, associated with the wall of the enteron, is called visceral or splanchnic. The now capa- cious cavity resulting from expansion of the pouch is a segment of the embryonic body- cavity or coelom. The myotome rapidly thickens and also increases its dorsoventral extent
. The chordates. Chordata. Reproduction 269 MC contiguous to the endoderm. The thick part is destined to form a seg- ment of body-muscle and is therefore called the myotome (Fig. 223, M). The outer layer, being, in con- junction with the ectoderm, the body-wall of the embryo, is called the somatic or parietal layer. The inner layer, associated with the wall of the enteron, is called visceral or splanchnic. The now capa- cious cavity resulting from expansion of the pouch is a segment of the embryonic body- cavity or coelom. The myotome rapidly thickens and also increases its dorsoventral extent. As it thickens, the adjacent upper portion of the coelomic space is correspondingly reduced. Eventually the somatic and visceral layers become joined by a horizontal septum formed just below the myotome (Fig. 223). Consequently, a lower major part of the original coelomic space is separated from an upper remnant of it, the myocoele (MC), which, with continued expansion of the myotome, is finally obliterated, while only the lower cavity participates in forming the definitive coelom (C). The thin portion of the wall of the myocoele later gives rise to connective tissue including the. Fig. 223. Amphioxus. Transverse section midway of the length of the body of a larva with five gill-clefts. (C) Coelom; (EC) ectoderm; (EN) endoderm; (I) intes- tine; (M) myotome; (MC) myocoele; (NC) notochord; (NT) neural tube; (V) sub- intestinal vein. (After Hats- chek. Courtesy, Neal and Rand: " Chordate Anat- omy," Philadelphia, The Rlakiston Company.) myoseptums (myocommas), which in tervene between and tie together successive segments of muscle. As a result of the general expansion of the mesodermal layers, not only, as stated above, are the walls of right and left pouches brought together in the mid ventral region, but the adjacent walls of successive pouches on the same side of the embryo become closely pressed to- gether. At this stage, then, the paired coelomic spaces of the sev
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