206 BIOLOGY: GENERAL AND MEDICAL 



II. Type. Meroblastic eggs (with yolks). 



Partial cleavage: a. Discoidal cleavage (fishes, 

 birds, and reptiles), b. Superficial cleavage (in- 

 sects and arthropods). 



Thus it is at once apparent that the presence or ab- 

 sence of a considerable yolk determines whether the 

 cleavage shall be total or partial, and the examination 

 of any thorough description of the development of the 

 chick will make clear the manner in which the enormous 

 yolk modifies segmentation. 



So soon as segmentation has begun it is possible to 

 recognize a chief or primary axis and to differentiate 

 an animal pole and a vegetative pole. Those cells that 

 arise in the neighborhood of the animal pole give rise 

 to the ectoderm from which the integument, the nervous 

 system, the glands; the organs of special sense, etc., 

 develop, which, so to speak, preside over the animal 

 function. Those from the opposite pole comprise the 

 cells of the entoderm } from which arise the digestive and 

 reproductive organs which preside over the vegetative 

 functions. 



This chief axis may be recognized, in large mero- 

 blastic eggs, even before development begins, and in 

 many cases is distinct as soon as it begins; thus, in the 

 hen's egg, the germinal vesicle represents the animal 

 pole, the great opposed mass of the yolk the opposite or 

 vegetative pole. 



All of the metazoa early show a monaxial, hetero- 

 polar condition about which the developmental process 

 centres. In eggs with yolks the animal cells are lighter 

 than the vegetative cells, so that the animal pole always 

 turns up, no matter in what position the egg is placed. 

 In eggs without yolks and with equal or fairly equal 

 cleavage the animal cells distribute over the surface and 

 the vegetative cells arise within, so that the vegetative 

 pole is central. 



The process of cleavage takes place through karyo- 

 kinesis, the plane of division being perpendicular to the 

 long axis of the spindle. Two cells are thus produced, 



