DEVELOPMENT AND LIFE-HISTOKIES OF TELEOSTEAN FISHES. 755 



of the first month the young embryo shows little further modification. The development 

 of translucent spicular plates upon the surface of the more exposed bones, especially of the 

 face, is a noteworthy feature in the young fish, but belongs to the post-larval period. 

 It may be noted, however, that these homogeneous spicular plates are not solely of 

 dermal origin — in fact, they develop as a thin outer layer of the true cartilaginous 

 elements, and arise within the nucleated perichondrium. 



The connective-tissue septa of certain muscles which are much used, become changed 

 into thin rods of clear spicular substance, a median rod, for example, passing up beneath 

 the pericardial cavity, and forming a fulcrum for the retractor hyoidei muscles. A similar 

 bar occurs also between the genio-hyoidei. 



Brain. — The anterior enlarged portion of the neurochord, of which frequent mention 

 has already been made, is really the brain. It extends the whole depth of the fore-region 

 of the embryo, forming a somewhat rhomboidal mass, rounded above, deeply carinate 

 below, and arched over by the epiblastic integument, while it is limited ventrally by the 

 hypoblast (PL IV. figs. 3, 4). The growth of the large optic vesicles, as two massive 

 ellipsoidal bodies (op, PL IV. figs. 14, 16), protruding laterally from this region, is an 

 early and notable feature ; but the details will be considered later, with the sensory organs. 

 The part which becomes the mid-brain (mb, PL IV. figs. 16, 17) is very early distinguished 

 by its greater breadth and volume from the narrower and prominent snout (fb), while the 

 hind or metencephalic part (hb) gradually passes away into the neurochord (ne) of the 

 trunk. No division as yet separates the encephalic from the spinal portion of the neuro- 

 chord, and the former is distinguished only by its increased breadth and depth. It is 

 remarkable, too, as extending fully one-third the total length of the embryo in its early 

 condition. No transverse cerebral folds appear until about four-fifths of the yolk are 

 enveloped, when a cleft, very obliquely directed, appears on each side of the post-optic 

 region. An anterior portion — the united mid- and fore-brain — can now be distin- 

 guished from the hind-brain (hb). The latter is, very shortly after, separated by a 

 similar though less marked fold from the nervous cord (ne) behind. Lereboullet 

 noticed this early transverse folding, which he says is due to the brain becoming doubled 

 upon itself; but he erroneously supposed that the cleft first formed is the metencephalic, 

 instead of the mesencephalic, and further conceived the neural tract to consist of two 

 parallel tubes. These becoming folded, produce two vertical projections, which he calls 

 the cerebellar lamellae (No. 93, p. 533). It is really the cerebral fold, the cerebellum 

 being, as just stated, marked off slightly later. The mid-brain, lastly, is constricted off 

 by an interorbital fold, so to speak, and the three regions of the brain are now defined 

 (PL IV. fig. 1 7). Reference has already been made to the dorsal or medullary groove ; 

 but it is at this stage, when the brain is separable into cerebellum and united mid- and 

 fore-brain, that this groove often appears in a very marked manner. Thus, in T. gurnardus, 

 on the fourth day, a deep median fissure may be seen — the sides of which slope at an angle 

 of about 70°. It is a temporary groove, as previously pointed out, and not apparently con- 

 nected with the subsequent cerebro-spinal canal. Soon after the closure of the blastopore, 



