SEGMENTATION 



13 



in an egg in which the proportion of yolk. present approaches the 

 maximum. In this case the zygote nucleus commonly undergoes 

 two mitotic divisions before 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 but not 

 extending to its periphery (Fig. 8, A). Occasionally a second regular 

 furrow makes its appearance intersecting the first at right angles 



Fig. 7. — Vertical (meridional) sections through blastulae of Frog. (From Morgan, 1897.) 

 AR, commencing invagination ; 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 

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



It should be noticed in regard to these segmentation furrows 



