REPRODUCTION 



83 



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. Eventually the somatic and visceral layers 

 become joined by a horizontal septum formed just below the myotome 

 (Fig. 56). 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 connec- 

 tive tissue including the myocomnias which 

 intervene between and tie together successive 

 muscle segments. 



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 con- 

 sisting 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 

 cavities are all thrown into free communication to form one large space, 

 the definitive coelom, which finally shows no trace of its segmental origin. 



Consistently with the fact that the mesodermal pouches arise not 

 simultaneously but in sequence from anterior to posterior, differentiation 

 begins in the most anterior or oldest pouches and proceeds backward at a 

 rate depending on the age of the successively formed pouches. Thus, 

 the myotomes of the first pair of pouches may have differentiated so far 

 as to show the beginning of the development of muscle tissue while as 

 yet not more than ten of the eventual sixty-one pairs of somites have been 

 definitelv formed. 



Fig. 56.— AMPHIOXUS. 

 Transverse section midway of 

 the length of the body of a 

 larva with five gill clefts. 

 C, coelom; EC, ectoderm; 

 EN, endoderm; I, intestine; 

 M, myotome; MC, myocoel; 

 NC, notochord; AT, neural 

 tube; V, subintestinal vein. 

 (Modified from a figure by 

 Hatschek.) 



