594 



INVEETEBEATA 



CHAP. 



np 



parts of the larva, as evinced by the fact that the diameter decreases, 

 up till the end of the second day ; and the constituent cells of all the 

 tissues become smaller in transverse section. The left anterior 



coelomic pocket, or collar- 

 cavity, retains an open 

 connection with the arch- 

 enteron for a considerable 

 time, and the hinder ends 

 of the trunk-cavities also 

 remain open to the arch- 

 enteron for a considerable 

 time, whilst their anterior 

 portions are grooved off 

 from it. 



These anterior parts 

 become divided into 

 segments which are called 

 somites (som, Fig. 432). 

 The first pair of somites 

 are formed from the collar- 

 cavities. The somites are 

 at first small spherical 

 vesicles situated at the 

 dorso-lateral angles of the 

 gut. They have at first 

 only minute cavities. They 

 soon begin to grow down- 

 ward between gut and 

 ectoderm, and the cells 

 forming their walls become, 

 for the most part, thin 

 and flattened. An excep- 

 tion to this statement is 

 constituted by the cells 

 forming the inner walls of 

 the somites, where these 

 walls abut on the noto- 

 chord. These cells remain 

 columnar in cross-section, 

 and develop fibrils in their 

 cytoplasm, and so form 

 the longitudinal muscles 

 of the larva. 



When five or six 

 somites have been formed, 

 that portion of the arch- 

 enteron which lies in front of the anterior pockets, or collar-cavities, 

 becomes grooved off from the rest, and simultaneously divided 



FIG. 432. Transverse sections through an older 

 embryo of Amphioxus lanceolatus than that 

 represented in the previous figure, to show the 

 further development of nerve cord, notochord, and 

 coelom. 



A, anterior section, showing on the left the first somite 

 cut off from the front end of the trunk coelom, on the right 

 the collar-cavity. B, posterior section, showing the open 

 groove in which the trunk coelom terminates posteriorly, ch, 

 rudiment of notochord; col.coe, collar coelom; ./, neural 

 fold ; n.p, neural plate ; som, somite ; tr.coe, trunk coelom. 



