LATER EMBRYONIC DEVELOPMENT 



133 



(3) a second pair of diverticula, more posteriorly placed, which 

 continue to grow back towards the blastopore, and have paired 

 mesoblastic somites, the cavities in which are the beginnings of the 

 coelom in the body, constricted off from them successively from 

 before backwards (Fig. 85, A, usli) to form all the remaining 

 myotomes. 1 This is the first sign of segmentation in the animal, 







FIG. 85. Embryo of Amphioxus. A, in vertical section, slightly to the left of the 

 middle line. B, in horizontal section, ak, Ectoderm ; en, neurenteric canal ; dk 

 and ud, archenteron ; ik, endoderm ; mk, mesodermal folds ; n, medullary canal ; 

 us, first coelomic pouch ; ush, coelomic cavity ; V, anterior, H, posterior, end. 

 (From Korschelt and Heider, after Hatschek.) 



and at this stage, when it has about five pairs of mesoblastic 

 somites, it breaks out of its covering and becomes a free-swimming 

 larva. 



The mouth now appears, and soon grows to a large opening 

 on the left side of the now pointed anterior end (Fig. 86, A, m), 

 and the first gill-slit (ks) forms as a direct communication from the 

 front of the mesenteron (pharynx) to the exterior. It is ventral 

 at first, and then shifts over to the right side. 



The anus forms posteriorly, and the neurenteric canal closes 



1 The mesoblastic somites in Figs. 84 and 85 are all derivatives of the larger 

 posterior pair of coelomic pouches, the smaller more anterior ones not being shown. 

 For further details in regard to the coelomic pouches see MacBride, Quart. Journ. 

 Micr. Sci. xliii. p. 351, 1900. 



