522 EEPORT OF COMMISSIONER OF FISH AND FISHERIES. [68J 



the protean character of the means at the disposal of Nature to achieve 

 one and the same end. 



19. — The development of the lateral muscle plates and 



SOMITES. 



The lateral muscle-segments or somites of the body of the cod are 

 developed, as in other fishes, by the transverse segmentation of the 

 lateral muscle plates or somatopleures lying on either side of the neu- 

 rula. The first evidence of muscle plates in the embryos investigated 

 by me appeared about the tenth day, as represented in Fig. 23, pv. 

 They appear mainly in succession from before backwards, the first pair 

 developing a little way behind the auditory vesicles or the solid rudi- 

 ments of the latter. As development proceeds, however, the most an- 

 terior pairs of muscular segments are differentiated later than those 

 which first make their appearance on the sides of the body. As a rule, 

 however, it may be said that they are segmented oft' in succession from 

 before backwards towardsthe eud of the tail, in which they last appear. 

 A little while after the blastoderm of the cod's egg has closed, there are 

 about eighteen to twenty pairs of muscle-segments distinguishable in 

 the body of the embryo. In vertical transverse section across the body 

 they are at first triangular, with the inner concave face applied to the 

 side of the neurula. Like so many other portions of the embryonic fish 

 during these stages, they are quite solid, and in sections the only evi- 

 dence of a very well defined structure is their columnar stratum or ex- 

 ternal wall of cells. They are not all developed in the end of the tail 

 until that part of the embryo has been fully formed. In the progress 

 of the growth of the tail the muscular segments first appear in the 

 proximal portion or that with which the body is continuous. But in 

 the caudal region, after it has budded out, they have at first a differ- 

 ent form from that observable in the first muscular segments of the body. 

 They are here crescentic in transverse section, and not triangular as 

 they at first were in the body. They clasp the chorda, neurula, and a 

 ventral mesoblastic strand of cells, thus (|), on either side, the neurula 

 being uppermost, the chorda in the center, and the mesoblastic strand 

 of cells alluded to lowermost. At the tip of the tail, however, in its 

 early stages of outgrowth, the whole of these structures are absolutely 

 continuous ; that is, blended and lost in an apical mass of cells in which 

 no lines of demarkation can be made out. A little way forward the 

 lines of separation between these structures become apparent, as may 

 be seen in the tails of the embryos viewed from the side in Figs. 31 and 

 32. After the outgrowth of the tail the embryo's body has grown very 

 notably in vertical thickness, upon which the muscle-segments of the 

 body begin to assume the crescentic form seen on either side of the tail, 

 except that in sections of the anterior regions the ventral limb of the 

 crescentic muscular segments are truncated, resting with their blunt 

 ends upon the splanchnopleure. With the growth of the body the mus- 



