14 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. 



that the first latitudinal furrow cannot be identified in the Elasmo- 

 b ranch and further that the study of sections shows that the furrows 

 sometimes cut into the germinal disc obliquely instead of being 

 perpendicular to the egg surface. 



The nuclei of the blastoderm divide synchronously and after four 

 di \isions have taken place, when there are sixteen nuclei in place of 

 the original single zygote nucleus, the segmentation furrows (Fig. 8, 

 C) form a network dividing up the blastoderm into smaller central 

 and larger peripheral blastomeres. These blastouieres are, however, 

 not completely isolated from one another but are still in continuity 



B 



i-'n,. 8, Surface views or tin- blastoderm of Elanmobrancha illusti-Mtin- tin- ]>i 



of s,. -,,,,. ,,t ;l { i OI1 . (A I! IT kuekert, 1899.) 



D i -njiimn. H& B* showB an abortive fragmentation whioh 



',><! tiausitoi ilv loiind Un- iiiamin <>t the p-i niiii.-il disc | M.-nu-v ! se-tuent s ot 

 rf|. 



at tlu-ir bases, ami. in the rase of the ].<M i]>ln-ral Mast oincivs. a! tli.-ir 



(.llt.T (Mr 



l'|. In Ihr lil'lli lnitntic ilivisinFI lllc a\CS <!' lllr lllilnlic Spilldli'S 



have l-rn appnixiiiiMh-ly parallel to the surface Imt im\\ l.lasl(.nicivs 



begin 1 livil- \\illi lln-ii- spindlr axes perpendicular t<> llie surface 



K> thfi "f supel lieial seLHiienls 1). -ci ni les sepa I'a I e. I (ill'. I'.elieatll 



tliiid aceuiiiulatcs inlereelliilai'ly and a se^ineiitat inn cavity 



I-) 



Muring lli- -ixlh di\i-i"M .me nf the Mast miieivs 1'ni-iniii^ the 

 ii.-ntatinn Oavitj l.cn.nie sepai-aled .ill' IVnni the 



