1V LATER EMBRYONIC DEVELOPMENT Ip ys) 
(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, wsh) to form all the remaining 
myotomes.' This is the first sign of segmentation in the animal, 
Fic. 85.—Embryo of Amphioxus. A, in vertical section, slightly to the left of the 
middle line. B, in horizontal section. ak, Ectoderm ; cn, neurenteric canal; dk 
and ud, archenteron ; ik, endoderm ; mk, mesodermal folds ; », medullary canal ; 
us, first coelomic pouch; wsh, 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-sht (4s) 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. 
