422 



602 ANATOMY OF VERTEBRATES. 



a minutely granular matter, with clear corpuscles, vitelline cells, 

 and oil-particles : it projects from the surface of the yolk, fig. 

 422, a, and becomes transparent : the vitelline and oil-globules, 

 aggregating at its base, buoy it up. The formation of two 

 hyaline centres is followed by the cleavage of the germinal layer 

 into two equal parts, ib. b, and these are next cleft at right angles 

 into four, ib. c. In the Tench this occurred about half an hour 

 after the rising of the germ-layer. Each of the four divisions 

 undergoes subdivision, but irregularly, ib. d: further sub- 

 division gives the surface a mulberry character, ib. e, and 



finally the parts are broken up to 

 such a degree of minuteness that 

 the surface is ag-ain made smooth. 

 The hyaline principle, which is the 

 centre and cause of these successive 

 divisions, is thus diffused through, 

 or assimilated by, the whole germinal 

 First steps in the .irwiopcment of a Tench. layer, which has thereby become 



the ' germ-mass.' It now subsides 

 to the form of a circular disc, separated by a layer of oil-globules 

 from the yolk. The process occupies about three days in the 

 Salmon, and from fifteen to twenty hours in the Pike : l before it 

 is completed in the latter fish the yolk rotates within the ectosac. 2 

 A cavity is formed in the centre of the germ-mass, which, as 

 the mass subsides and extends over the yolk, is obliterated by the 

 contact of the outer and inner layers. It clothes half the yolk 

 by about the end of the third day, and when it covers two-thirds 

 or more, the rotation ceases. The margin of the germ-mass 

 encompassing the uninclosed part of the yolk is tumid. No 

 rotation takes place in the ovum of the Perch, 3 and the germ- 

 mass incloses the whole vitellus, as in the Cyprinoids. 



The peripheral layer in the Pike begins to rise from the tumid 

 margin of the germ-mass, as from a base, and extends, contracting, 

 towards the opposite pole : this tract of germ-substance is the e em- 

 bryonal ' or ' primitive trace.' It next sinks in along the median line, 

 forming a furrow, which stops short of the two ends of the trace : 

 that end opposite the point from which the germ began, swells into 

 the head, and the median furrow expands upon it ; the cephalic 

 borders are next united by a thin layer of epithelial cells above the 

 furrow, converting it into a cavity or ventricle, and the myelonal 

 furrow is similarly covered by a layer, uniting the lateral columns. 

 The embryonal trace becomes longer, narrower, and bends round 



1 cccxix. p. 486. - cxxxt. * Ib. p. 512. 



