DEVELOPEMENT OF FISHES. 



603 



half the vitellus, fig. 422,/. 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 

 stria? 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 ' notochord,' 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. f, and these have now 

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



Head of embryo Pike, cccxix. 



