8 EMBKYOLOGY 0* THE U>\VKi; VERTEBRATES GH. 



end until finally it forms a wide meridional valley encircling the 

 entire egg (Fig. 3, B). This valley gradually deepens dividing the 

 egg into two halves. Finally after about 5 minutes from the 

 commencement of the process the protoplasmic bridge connecting 

 the two halves snaps across and the egg is now completely divided 



^ 



. .;. Illustrating tin- proci's> of s.-im-nUti.ui >!' tin- 1-1:1; <>t .!////<// / 



(Aft.-r Hatsd.ek, 1881.) 

 Th<- :i|.ir:il ]...].- j> :il,o\c in i-:iHi li^nir. Th,. .s.-r.-ml |,.,I;n Ix.ilv is si't-n in prMximily t> it. 



into t\\ Mastnmeres, each ot \\hieli assumes a splierieal shape in 



.il.K-r tension. The l\\< |.la.slnmeres he.-i.nie slightly 



tl.iltein-d \sheiv Lln-v ;iiv in euntaet i.e. in 1 he plain- <>!' I he iirst 



iMilial flllTMU Fig, :',, C). Tin- lllture Cdiirse nf de\ -elnpnu-ilt 



nhoWH that this plane (-(.nvsj.onds tn the si^il lal J'lane f the 



enihryo (< Vrl'uiil a i ne. IIMHJ): in nther \\nrds the t\\ 1 ila^l mm-ivs 



represent ih- ri.:_ r lii and i.-n halves of the developing individual, 



