DEVELOPMENT OF ELASMOBRAXCH FISHES. 149 



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-plate 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 be- 

 comes divided up into a number of fibrillae. 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 

 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 (PL XYii. fig. 1). Small portions of several 

 muscle-plates come in this way to be situated in the limbs, 

 and are very soon segmented off from the remainder of the 

 muscle-plates. The portions of muscle-plates thus introduced 

 into the limbs soon lose their original distinctness, and can no 



