' i i"\ 0V -MM: -I I DUCTS. 



11 



dt siirnat d /nrnmtirc 

 oth.r initi'itif' 



A.I' 



cleavage, m:.k- tlnir appearance only at the region ol 



which is rich, r in |n>i.>pla~m, wheitai the region i more 



voluminous :,nd richer in d utoplasin remains appan t. ! 



unalttnd, and U DOi divided np into nils. Uy tl. 



contn.M. \\hich \ nt in tin- '.<! egg, 



mis durini: ! M Nipim nt disproport ionately | 

 ohviors. Tin- on* part und di\id- d into . 



out of tlie.se products the individual . ' he ot In r par 



more or 1< s- undtend, and is gradually empl' 

 material. Kollowini: the example of l;i:n IM:I:T. the part >' 

 yolk which is richer in pi otopl.-.sm, and to which the d v.-lopmeii- 

 tal pioc. BBl s remain 

 has h 



yolk, and the 

 yolk. 



The unequal distrihutioii of 



folk (viteUtu ''<>////"- 

 -) and of itntritii-r ; /olk 

 HHfi'ffii'iix) within the 

 \S accomplished in t \\o dif- 

 ferent W3JB. 



In the ' tii:. 3) the 



formative yolk is accumulated 

 at one pole of the Bgg BD a //a/ 

 tjt'rin -1/ixr (/.'.. vr/r ). Inasmuch as 

 its specific irravitv i- lcs> than 

 that of the nutritive yolk (n.d) 

 collected at the opposite pole, it 

 is always directed upward, and 



it spreads itself out on the yolk just like a drop of oil on water. In 

 this case, therefore, the ei::: has under-one a pol 

 when at rest it must always assume a definite poatian, OW1I 

 unequal wei-ht of the two pol- 



g*Med: tl,,- <>/>/ r. //////// ^>l>\ *///// // -' animal 



(A./'): f/<> under, heavier and rtfllar in /// 



(!'./'). The polar ditlen-ntiatioii -f e--s i Mcountcivd in 



Vertelu-ates. and i- e-pecially prominent in the classes of Bony 

 Fishes. Reptiles, and Uirds. 



In the Moond OM (ti-. 4) ' ntlated 



///..- t rlt,,l.' surl'ttce / tin- '.'/:/. and surrounds ti .lly placed 



nutritive yolk (/</) Ofl a uniformly thick, tinely -rranular cortical 



Fiy. 3. -Diagram of an egg with the nutritive 

 yolk in a polar position. I 



\<ilk <-i'H>titut<-s at tin- animal \*<\<- 

 germ-disc (k.tfh), in \\ln.-h ti 



tills tin- iv .-t -f the egg up 

 vegetative pole ( I 



