74 EMBRYOLOGY. 



1. Discoidal Cleavage. 



This takes place in eggs with nutritive yolk in a polar position 

 The process of cleavage remains confined to the formative yolk 

 accumulated at the animal pole, which has the form of a disc and 

 contains only a small amount of deutoplasm. There is formed, con- 

 sequently, a cellular disc. (Fishes, Reptiles, Birds.) 



2. Superficial Cleavage. 



This occurs in the case of eggs with central yolk. In typical 

 cases the nucleus alone, which occupies the middle of the egg, under- 

 goes repeated division. The numerous daughter-nuclei which arise 

 in this manner migrate into the layer of protoplasm which invests 

 the central nutritive yolk, and the protoplasm is thereupon divided 

 into as many segments as there are nuclei lying in it. There is 

 formed a germ-membrane (Keimhaut). (Arthropods.) 



7. Eggs with total cleavage are designated as holoblastic, eggs 

 with partial cleavage as meroblastic. 



8. The direction and position of the first cleavage-plane are strictly 

 conformable to laws which are founded in the organisation of the 

 cell ; they are determined by the following three factors : 



First factor. The cleavage-plane always divides the axis of the 

 nucleus which is preparing for division perpendicularly at its middle. 



Second factor. The position of the axis of the nucleus during 

 division is dependent upon the form and differentiation of the en- 

 veloping protoplasm. 



In a protoplasmic sphere the axis of the nuclear spindle, occupying 

 the centre of the sphere, can lie in the direction of any radius what- 

 ever ; but in an oval protoplasmic body, only in the longest diameter. 

 In a circular disc the nuclear axis lies parallel to its surface in 

 any diameter of the circle, but in an oval disc only in the longest 

 diameter. 



Third factor. In the case of eggs of unequal segmentation, which, 

 in consequence of their unequally distributed, polar deutoplasm, 

 are geocentric, and therefore assume when in equilibrium a parti- 

 cular position, the first two planes of cleavage must be vertical, and 

 the third must be horizontal and placed above the equator of the 

 sphere. 



