THE DEVELOPMENT OF AMPHIOXUS 31 



of the right, e.g., when there are eight somites on the left there 

 are but seven on the right. 



The enlargement and ventral elongation of the somites men- 

 tioned above is accompanied by a differentiation of their walls. 

 That part of the somite lying just beneath and in contact with 

 the ectoderm becomes thinner and forms what is known as the 

 parietal wall of the somite, or the somatic mesoderm (Fig. 8, E) ; 

 the region in contact with the wall of the gut or enteron also 

 becomes thinner and forms the visceral wall or splanchnic meso- 

 derm: while the more restricted portion bordering the chorda 

 thickens by the horizontal flattening and antero-posterior 

 elongation of the cells through the extent of the somite. This 

 last region of the somite is called the myotome, while the somatic 

 and splanchnic regions together are termed the lateral plate. 

 In the myotome the cells begin, as early as the nine-somite stage, 

 to differentiate as muscle cells (Fig. 8, E) forming a muscular 

 epithelium. At the close of the embryonic period the more 

 anterior my o tomes are capable of muscular contractions. 



The thinning of the parietal and visceral walls of the somite 

 and their downward extension, considerably enlarge the en- 

 closed cavity and carry it down around the sides of the enteron. 

 The cavities of the somites now become recognizable as the 

 beginnings of the ccelomic space. The enlargement of the myo- 

 tome partially obliterates the cavity of the somite in the dorsal 

 region, the small cavity remaining being termed the myoccel. 

 The larger cavity of the lateral plate is then distinguished from 

 the myocoel by the term splanchnoccel. Before the close of the 

 embryonic period the ventral walls of the more anterior somites 

 push completely around the enteron and meet in the mid- 

 ventral region, separating the enteron from contact with the 

 ectoderm. Presently the median walls of the splanchnoccels 

 for the most part disappear, and the splanchnoccels of each pair 

 of somites fuse more or less completely. Remnants of these 

 median partitions below the enteron appear to remain as the 

 rudiments of the subintestinal vein and branchial artery. 

 Finally the somite becomes divided by a horizontal partition 

 between the lateral plate and myotome, and the corresponding 



