DEVELOPEMENT OF FISHES. 



603 



423 



half the vitellus, fig. 422, f. A layer of epithelial cells forms a 

 net-work over the whole dorsal (upper) surface of the embryo. 

 In the germ-mass broadening from the primitive trace oblique 

 striaB appear, indicating its division into segments : these begin- 

 nings of aponeurotic septa probably accompany and support ner- 

 vous productions from the myelonal columns. 



Two transverse constrictions begin to divide the cephalic 

 enlargement into three lobes, the second and third of which 

 expand into vesicles : an accumulation of cells at the sides of the 

 middle expansion appears to add greatly to its breadth, but forms 

 the basis of the eyes. A similar accumulation of darkish granular 

 matter on each side of the third enlargement lays the foundation 

 of the acoustic vesicles. 



The differentiation and confluence of the cell-constituents of 

 the primitive trace have previously led to the formation of a 

 pair of albuminous chords along the sides of the median furrow, 

 forming the myelon proper ; the cells exterior* to and above them 

 are converted into muscle and fibrous septa, whilst beneath the 

 columns is the jelly-filled cylinder, with a transversely striate 

 sheath, pointed at both ends, forming the ( iiotochord,' fig. 423, ch : 

 its anterior point passes a little 

 in advance of the acoustic vesi- 

 cles, ib. f. Beneath the noto- 

 chord and surrounding blastema 

 is stretched the vegetative or 

 mucous layer of cells, in contact 

 with the yolk. Both head and 

 tail of the now cylindrical em- 

 bryo are liberated from the sur- 

 face of the yolk. A fold of 

 blastema, reflected from the under part of the head, sinks, like a 

 pouch, ib. /, into the yolk, and soon includes the rudiment of the 

 heart, like a bent cord, ib. k, which begins to oscillate about the 

 seventh day. From the mid-line of the inferior surface of the 

 embryo, or its mucous layer, two longitudinal plates descend, 

 diverging into the yolk-substance, and form the primitive intes- 

 tinal groove. 



The ophthalmic vesicle, ib. g, elongates and curves outward, until 

 the two ends almost come into contact : between those ends and 

 beneath the delicate tegumentary layer connecting them the 

 crystalline lens, ib. b, is formed. About the same time, the 

 otolites appear in the acoustic vesicles, ib./*, and these have now 

 acquired a cartilaginous case. The cerebral lobes, r, begin to 



Head of embryo Pike, cccxix. 



