524 REPORT OF COMMISSIONER OF FlSff^SmffTSHimES. [70] 



cle-pjates during development, the appearance of muscular cones in two 

 lateral series, one above and one below the middle line of the side. 

 These constitute the dorsolateral and ventro-lateral systems of muscu- 

 lar plates. External to the dorso-lateral and ventro-lateral plates, dif- 

 ferentiated as above described, on the middle of the sides, there is 

 developed a thin strip of muscle, which in some adult fishes is quite 

 distinct, especially when they are boiled, when it appears as a dart 

 muscular band, in striking contrast with the white substance of the 

 muscular plates or cones. These outermost plates of dark-colored mus- 

 cle are developed during a late larval stage, and appear to be derived 

 by delamiuation from the same somites from which the dorso-lateral 

 and ventro-lateral plates have been differentiated. These dark lateral 

 bands of muscle in the adult are segmented the same as the deeper 

 plates, and their segments correspond in number and segmental position 

 to the latter. 



A very great difference in the downward extent of the muscle plates 

 over the sides of the yelk-sack, is manifested m the embryos of differ- 

 ent species of the same age. In the salmon and white-fish, the muscle 

 segments extend for a considerable distance over the yelk-sack at the 

 time of hatching, and after a variable time they completely inclose what 

 remains of it. This is due partially to the collapse of the yelk, as it is 

 absorbed, and partially to a downgrowth of the muscle-segments over 

 the sides of the sack, between the epiblastic and splanchuopleural layers, 

 the latter being carried along with the growth of the muscular layer. 

 In other forms the whole of the sack may be absorbed before there is 

 the least tendency of the muscle-segments to grow down and inclose it 

 by their ventral borders. This is a noteworthy characteristic of the em- 

 bryos of the shad, cod, Spanish mackerel, and other species, and again 

 illustrates the singular way in which the young of a relatively homoge- 

 neous group may differ from one another. In the salmon the ventral de- 

 velopment of the muscle-segments seems to be hastened ; in the other 

 cases it is evidently retarded. In the cod, for example, the edges of the 

 muscle-plates do not even reach down so far as to cover even the upper 

 lateral portion of the remains of the yelk-sack, as may be noticed in 

 Fig. 49. 



The material for the development of the posterior muscle segments of 

 the embryo is also supplied in a singular way. In Tylosurus there is 

 evidence of the concrescence of the rim of the blastoderm at the tail end 

 of the developing axis of the embryo as may be surmised from an in- 

 spection of Fig. 6, pi. XIX, which accompanies my essay on the devel- 

 opment of that form. The rim In this case does leave the embryonic 

 axis at right angles on either side, as in the cod, as shown in Figs. 19 

 and 20 of this memoir, but at an obtuse angle after the time for the 

 closure of the blastoderm is approaching. While there is a veritable 

 caudal swelling it is also manifest that a veritable concrescence of the 

 rim of the blastoderm is taking place by intussusception or gradual 



