[69] E^IBRYOGRAX'nY OF OS^EOTTS FISHES. 523 



cle-segments also inci^ease in volume, and a perceptible increase in their 

 length and width also takes place, as may be seen upon comparing their 

 dimensions as shown at jw in Fig. 31 with those represented in Fig. 32. 

 A very remarkable metamorphosis of their cells now begins to take 

 place, by which they become stretched out as muscle-cells which cor- 

 respond in length with the segments themselves. These muscle-cells 

 also soon become transversely striated like voluntary muscle fibers gen- 

 erally, and have a distinct oval nucleus imbedded in their medullary 

 substance, which may be very nicely demonstrated by the use of borax 

 carmine. The primitive fibers also soon split up by processes of divis- 

 ion into fibrils which are arranged in bundles, the fibrils themselves 

 appearing in transverse sections as if they were arranged around a cen- 

 tral empty space. With the progress of development, however, still 

 other clmnges of form of the muscle-plates as wholes occur, and the 

 first of these to be apparent is the > -shaped form they assume when 

 viewed from the side of the body. They are then arranged thus > > > > 

 in succession on either side of the body. The development of this last 

 feature also proceeds from before backwards, being more marked at 

 the anterior end of the body than at the posterior at an early stage. 

 Still aijotber point may be alluded to here relating to their arrange- 

 ment : with the advance of development the anterior and posterior edges 

 of the muscle-segments also become more and more beveled, the bevel 

 trending backwards. On this account they finally overlap each other; 

 that is, the hinder beveled margin of one segment covers the anterior 

 margin of the succeeding one, which has its front edge beveled in the 

 opposite direction. During the later embryonic stages still other changes 

 occur in the relation and form of the segments themselves, when a 

 smaller < -shaped portion is developed at the upper and lower margins 

 of the individual plates, which open forwards instead of backwards, the 

 reverse of the middle > , but which fit into each other in the same 

 manner. The foregoing constitute the main features of the metamor- 

 phosis of the lateral muscle plates of the larval fish into those of the 

 adult, and relate altogether to changes of form and histological consti- 

 tution. 



Another series of changes also occur, which effect the arrangement 

 of the mnscular plates into a dorso- ventral, lateral system. This is the 

 division of the muscle-plates into two superimposed masses on either 

 side, by the development of a horizontal ligamentous septum along the 

 middle of the sides, and extending inward almost from the skin to the 

 vertebral column. This lateral septum is continuous on its upper and 

 lower sides with the intermuscular ligaments placed between the sin- 

 gle pairs of muscular somites, producing the remarkable appearance of 

 systems of rings of muscular tissue, arranged in a ventral and dorsal 

 position on either side of the vertebral column, and well seen in a frozen 

 section of the tail of an adult fish. The lateral and intermuscular septa 

 produce, together with the peculiar bending and beveling of the mus- 



