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 several 

 pouches are separated from one another by thin partitions, some trans- 

 verse and others median, each consisting of two layers of cells. These 

 partitions become progressively thinner until they perforate and finally 

 disappear completely except that remnants of the median ventral wall 

 may persist in connection with the development of blood-vessels. With 

 the obliteration of these partitions, the several segmentally developed 

 coelomic cavities are all thrown into free communication to form one 

 large space, the definitive coelom, which finally shows no trace of its 

 segmental origin. 



