THE PROCESS OF CLEAVAGE. 73 



4. The division of the nucleus takes place in the following manner : 

 the nuclear filaments split lengthwise, and their halves move apart 

 in opposite directions toward the ends of the spindle, and are there 

 converted into vesicular daughter-nuclei. 



5. The protoplasm arranges itself around the ends of the spindle 

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

 a double radiation or an amphiaster arises in the egg. 



6. The external phenomena of cleavage consist in the division of 

 the egg-contents into individual parts, the number of which corre- 

 sponds to that of the daughter-nuclei. They exhibit various modifica- 

 tions, which are dependent on the arrangement and 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. (Holoblastic eggs.) 



The eggs, which for the most part are small, contain a small or 

 moderate amount of deutoplasm, and are completely divided into 

 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 general, are of uniform size. (Amphioxus, Mam- 

 malia.) 



2. Unequal Cleavage. 



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

 equally distributed, being concentrated toward the vegetative pole, 

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

 more protoplasmic pole. Usually the segments become unequal in 

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

 Amphibia .) 



II. Partial Cleavage. (Meroblastic eggs.) 



The eggs, which are often very large, ordinarily contain con- 

 siderable quantities of deutoplasm. In consequence of the unequal 

 distribution of this, the egg-contents are separated into a formative 

 yolk, in which alone the process of cleavage is manifested, and a 

 nutritive yolk, which remains undivided, and is used up during 

 embryonic development for the growth of the organs. 



