6o 



CHORDATE ANATOMY 



-EC 



-NC 



-M 



-MC 



-EN 



while elsewhere the wall remains relatively thin. The expansion of the 

 pouches continues until the walls of right and left pouches meet in the 

 median plane beneath the enteric endoderm. At this stage three regions 

 of the mesoderm may be distinguished: the thickened part lying alongside 

 the notochord; an outer thin layer contiguous to the ectoderm; and an 

 inner thin layer similarly contiguous to the endoderm. The thick part is 



destined to form a segment of body-muscle 

 and is therefore called the myotome (Fig. 

 52,M). The outer layer, being, in conjunction 

 with the ectoderm, the body-wall of the em- 

 bryo, is called the somatic or parietal layer. 

 The inner layer, associated with the wall of the 

 enteron, is called visceral or splanchnic. The 

 now capacious 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 dorso-ventral extent. As it 

 thickens, the adjacent upper portion of the 

 coelomic space is correspondingly reduced. 

 Pig. 72*X^MPHI0XUS. Eventually the somatic and visceral layers 

 Transverse section midway of become joined by a horizontal septum formed 



the length of the body of a . ^„. x r-^^ 



larva with five gill clefts, just below the myotome (Fig. 52). Con- 

 C, coelom; EC, ectoderm; sequently a lower major part of the original 



EN, endoderm; I, intestine; . . . j r ^^ 



M, myotome; MC, myocoei; coelomic space IS Separated from an upper 

 NC, notochord; NT, neural remnant of it, the myocoele (MC) which, with 



tube; V, subintestinal vein. . • r .^i ^ • £ „ii, 



(Modified from a figure by contmued expansion of the myotome, IS hnaliy 

 Hatschek.) obliterated, while only the lower cavity partici- 



pates in forming the definitive coelom (C). The thin portion of the wall 

 of the myocoele later gives rise to connective tissue including the 

 myoconmias which intervene between and tie together successive seg- 

 ments 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 together. 

 At this stage, then, the paired coelomic spaces of the several pouches 

 are separated from one another by thin partitions, some transverse and 

 others median, each consisting of two layers of cells. These partitions 

 become progressively thinner until they perforate and finally completely 

 disappear except that remnants of the median ventral wall may persist 

 in connection with the development of blood-vessels. With the oblitera- 

 tion of these partitions, the several segmentally developed coelomic 



