I in: ri:o<<E88 OP < 73 



4. Tin- division of tin- nucl-u> tak-> j.l., 



tin- Iiliclt-ar filaments split lrn.M liu i-. and their halvos mOVO :i 

 in opp it" direction^ toward tl, -idle, and an- i 



o>n\ertcd into \e>icular daughter-nuclei. 



The protoplasm arranges itself around the end> of the spindle 

 in filaments having the form of a stellate figure (an aster), so (hat 

 a double radiation or an amphiaster ari-e> in tin- egg. 



6. The external phenomena of cleavage con-i>t in the division of 

 tli" 'gg-contents into individual parts, the number of which corre- 

 sponds to that of the daughter-nuclei. Theyexhihit various modifica- 

 tion-, Avhii-h are dependent on the arrangement an< I distribution of 

 the egg-plasm and the deutoplasm, as is to be seen from the fol- 

 lowing scheme of segmentation. 



Scheme of the Various Modifications of the Process 



of Cleavage. 

 I. Total Cleavage. (Iloloblastic e- 



The eggs, which for the mo.st part are small, contain a small or 

 moderate amount of deutoplasm, and are completely divided 

 daughter-cells. 



1. Equal Cleavage. 



This takes place in eggs with meagre and uniformly distributed 

 deutoplasm (alecithal). By the process of cleavage there are formed 

 segments which, in grnoral, are of uniform size. (Amphioxus, Mam- 

 malia.) 



2. Unequal Cleavage. 



This occurs in eggs in which a more abundant dentoplftsm is un- 

 equally distributed, being concentrated toward th 

 and in which the cleavage-nucleus is located nearer the animal and 

 more protoplasmic pole. Usually the segments become umqual in 

 size only with and after the third act of division. (Cyclostf 

 Amphibia.) 



II. Partial Cleavage. < Mrrohlastic rgL r -.) 



The eggs, which are often very largr. ordinal ily contain con- 

 siderable quantities of deutoplasm. In consequence of the unequal 

 distribution of this, the egg-contents are separat* d into a formativo 

 yolk, in which alone tlu- proci ss of cleav.-^'- i- in.xi.it'' >N d. and a 

 nutritive yolk, which remains undivid.-d, and i> used up during 

 embryonic development for the growth of the organs. 



