mi: PROI M 



Weismann und Ischikawa. \\Viteiv I'm- 



dt-r i:ii'li1uiii,'k".rpiT. /...!. .lahrl.iicli.T. IM. III. \ Mrph. 



,p, 516. 



Weismann und Ischikawa. l.Vbn- die Pnracopnlation im I>:ipl,iii. 

 sou i.- liner iii'it'iuiu ii. Befmohtang dflsselboi. /"i. .iai.ii>ti<-h. i. i:<i. iv 



A bill. f. M< rp 

 Whitman, C. O. 'I h' Kinetic I'ln-iMiiP n:i of tin- K_ru r dun ML; .^ a I ur.it in ;in<! 



ii'latii'ii. .lour. Mi.iphol. \'-l. I. 1 

 Zaoharias, Otto. NVue rntrrHirhui^vn iiU-r !! Copnlntinn der Ge- 



.srhlreht.spr.Mliicte unil dm Mrt'nicl.tui. negalo- 



rrphala. An-liiv f. inikr. Anat . II.I. XXX. 17 

 Zacharias,Otto. l>i<- IVim-ivn Vor-iinuc b t -i d<-r i'.ct'nu-htiin;.; cheii 



Eics. J5i.,l. LVntralblatt. 1'.<1. VII. 1888, p. ;:,'.. 



CHAPTER III. 

 THE PROCESS OF CLBA \'\<;r.. 



FERTILISATION is in most instances immediately follmvo<l lv further 

 development, which begins with the division of the ci:^ cell the 

 Minplf I'liMiK-ntnry orgnnism into an ever-iiiciv.-isiii^ nuiiihcr of 

 small cells the process of cleavage. We shall Iv-in the >tudyof 

 cleavaire with a very simple case, and here also choose as a foundati< n 

 for the presentation of the suhjtct the e^ <>f an KchinMuVnn and 

 the vg^ of the common Ascaris of the 1 1 



In the living ei. r tf of the Echinoderm the deftvage-niioleua (!iir. L') 

 //:), \\hich arose from the fusion of egg-nucleus ami >jennatic 

 nucleus, is at tir^t spheroidal, and lie> exactly in the middle of the 

 \\hero it forms the centre of a radiation which a fleets the 

 whole yolk-mass; but it soon begins to be slightly elongate.', and 

 at the >amo time to become less and le.-s distinct, -oth.-it \\itl; the 

 living object one might be milled into asMiming that it had heen 

 rompleU'ly di->..lvrd. IJefore thi<. 76TJ regular ehangea in the dis- 

 tribution and arrangement of the piotopla>m around the nucleus 

 have taken place. The mODOOentrio I'adiation resulting from fer- 

 tilisation is dixi.lel. The two i rwly t'orm..! ra, li.it i -iipon 

 move to the poke Of the i-longitel uu lens. At fir.-t Miiall and in- 

 significant, they rapidly \te:. d, ami finally each oeeiipies a half of 

 the egg (tig. L'7). and the rays of the two >ysteii.- I a .sharp 

 angle in the median plane of the 



Jn>t in proportion as the two radiations become more di>'.inct, 

 there arises, within the granular yolk, a> the omt 



