218 BIOLOGY: GENERAL AND MEDICAL 



develop according to the plan later to be described, di- 

 vides so as to form a long chain of morulae, from each of 

 which a separate embryo develops. 



Polyembryony also appears occasionally among animals 

 as high as mammals. The researches of Fernandez and of 

 Neuman and Patterson have shown that each fertilized 

 ovum of the armadillo produces four embryos. Here the 

 polyembryony is regular and constant. In man the occa- 

 sional occurrence of monochorionic twins (homologous 

 twins) from the development of a single ovum may be con- 

 sidered an "accidental" occurrence of polyembryony. 



The mode of segmentation differs in different eggs, 

 partly through peculiarities of the eggs, partly through 

 inherited impulses. Hertwig presents the following 

 scheme of cleavage: 



I. Type. Holoblastic eggs (without yolks). 



Total cleavage: a. Equal cleavage (lower inverte- 

 brates and mammals), b. Unequal cleavage (mol- 

 lusk and amphibia). 



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 



