EARLY LARVAL DEVELOPMENT. 547 



can be followed forward from the posterior end of the body as a clear 

 canal (Figs. 'J**, L ) - S '.. sv). Near the second segment it is deflected 

 to the right side of the body by the first gill-rudiment forming in the 

 ventral median line, and ends, apparently blindly, near the club- 

 shaped gland. The origin of this vessel is still obscure. KOWALEVSK* 

 derives it from cells lying free in the body-cavity which form a solid 

 strand that becomes hollowed out later. 



As the cavities of the primitive segments gradually increase in size 

 the cells of their walls become more and more flattened. This does 

 not apply, however, to the cells in contact with the sides of the noto- 

 diord which, on the contrary, become greatly elongated as columnal 

 or club-shaped cells and form the rudiment of the lateral trunk-muscles 

 (Fig. 284). These cells contain nuclei in their free club-shaped ends, 

 while the basal portion, that is turned to the chorda, becomes trans- 

 formed into muscular tissue. Kach of these muscle-cells traverses the 



np 



Fi<i. 287. Stilly of .I/////// /<(.<:;//> with thirteen primitive segments (after HATSCHKK). 

 The liouinlnrif- of the segments of the left side are indicated by uninterrupted lines, 

 and those on the right by dotted lines, en, ueurenteric canal ; dr, rudiment of the 

 club-shaped -il-unl. >'>'. riirlit. <ir" ', left, anterior entoderm-diverticulum ; np, 

 nenropore. 



whole length of the primitive segment to which it belongs (Fig. 286, 

 in-.), and its axis lies in a line with the corresponding cells of the 

 neighbouring primitive segments. The muscle-fibres formed by these 

 cells fuse together and run continuously through the consecutive 

 segments. Each muscle-fibre is thus formed from a series of con- 

 secutive muscle-cells. Transverse striation can early be made out 

 in the fibres. The fact that the walls of the primary gut-pouches 

 (enterocoeles) give rise not only to the lining of the body-cavity but 

 also to the trunk-muscles, has caused some observers to speak of the 

 primary cavities as the myocoelomic pouches. At an early period 

 after the ventral extension of the mesodermal somite described above, 

 its cavity becomes divided into two by the formation of a horizontal 

 partition which separates a ventral splanchnocoele from a dorsal 

 myocoele (see pp. 564 and ~>65). 



