SEGMENTATION 



in an r-j^ in which the proportion of yolk present approaches the 

 maximum. In this case tin- /y^ote nucleus commonly undergoes 

 two mitolie divisions be-fmv there is any external symptom of 

 segmentation of the cytoplasm. Usually a single furrow makes its 

 appearance first, incising the surface of the germinal disc I ml m.t 

 r\ tending to its periphery (Fig. 8, A). Occasionally a second regular 

 furrow make- its appearance intersecting the first at right angles 



Ki<;. 7. Vertical (meridional) sections through blastulae of Frog. (From Morgan, 1897.) 

 l It, commencing imagination ; SG, segmentation cavity. 



and it is a curious point that it is sometimes this second furrow 

 which corresponds to the first nuclear division. 



These first two furrows apparently represent the ' first two 

 meridional furrows of the holoblastic egg though in the Elasmobranch 

 the first to appear may be either a or /3. More usually, in place of 

 a second regular furrow developing, irregular branches of the first 

 furrow, or even independent furrows, appear and an arrangement of 

 somewhat radiating furrows is brought about which gradually 

 converted into a network (Fig. 8, B, C, D). 



It should be noticed in regard to these segmentation furrows 



