3/O DEVELOPMENT OF ELASMOBRANCH FISHES. 



The remainder of the history of the muscle-plates presents 

 no points of special interest. 



Till the close of stage L, the muscle-plates are not distinctly 

 divided into dorsal and ventral segments, but this division, which 

 is so characteristic of the adult, commences to manifest itself 

 during stage M, and is quite completed in the succeeding stage. 

 It is effected by the appearance, nearly opposite the lateral line, 

 of a layer of connective tissue which divides the muscles on each 

 side into a dorso-lateral and ventro-lateral section. Even during 

 stage O the ends of the muscle-plates are formed of undiffer- 

 entiated columnar cells. The peculiar outlines of the inter- 

 muscular septa gradually appear during the later stages of 

 development, causing the well-known appearances of the mus- 

 cles in transverse sections, but require no special notice here. 



With reference to the histological features of the develop- 

 ment of the muscle-fibres, I have not pushed my investigations 

 very far. The primitive cells present the ordinary division, well 

 known since Remak, into a striated portion and a non-striated 

 portion, and in the latter a nucleus is to be seen which soon 

 undergoes division and gives rise to several nuclei in the non- 

 striated part, while the striated part of each cell becomes divided 

 up into a number of fibrilla:. I have not however determined 

 what exact relation the original cells hold to the eventual 

 primitive bundles, or anything with reference to the development 

 of the sarcolemma. 



The Muscles of the Limbs. These are formed during stage O 

 coincidently with the cartilaginous skeleton, in the form of two 

 bands of longitudinal fibres on the dorsal and ventral surfaces of 

 the limbs. Dr Kleinenberg first called my attention to the fact 

 that he had proved the limb-muscles in Lacerta to be derived 

 from the muscle-plates. This I at first believed did not hold 

 good for Elasmobranchs, but have since determined that it does 

 so. Between stages K and L the muscle-plates grow downwards 

 as far as the limbs and then turn outwards and grow into them 



a share in the formation of the great lateral muscles, which he denies to it. In an 

 earlier section of this Monograph, pp. 333, 334, too much stress was unintentionally 

 laid on the divergence of our views ; a divergence which appears to have, in part at 

 least, arisen, not from our observations being opposed, but from Dr Gotte's having 

 taken the highly differentiated Bombinator as his type instead of the less differentiated 

 Elasmobranch, 



