OEIGIN OF CELLS. 



19 



In the unimpregnated ovule spontaneous contractions are generally 

 seen in the protoplasmic cell-contentsr When the egg becomes fertilized 

 these contractions become so modified as to cause the germinal vesicle 

 (or the body resulting from the union of the male and female pronuclei), 

 and then the cell-contents to split into two parts contained within the 

 cell-membrane, which takes no part in this division. These segmentation 

 spheres, as already mentioned, continue to subdivide until an immense 

 number of minute, nucleated, membraneless cells are contained within the 

 vitelline membrane (forming the so-called mulberry mass). 



From these elements, which become progressively smaller as cleavage 

 goes on, all the tissues of the embryo are formed. At first they all 

 possess mobility (amoeboid movements), showing their analogy to the 

 simple amoeba, but at birth this property is only retained by certain cells. 

 Like amoebae, they also grow in size and divide into new individuals. 



At first all the cells resulting from the segmentation of the ovum 

 arei exactly alike: the}- then undergo certain modifications in arrange- 



12 3 4- 



Fig. 8.— Ova of the Dog from the Fallopian Tube, surrounded by the 

 Zona pellucida, in which are found numerous Spermatozoa, after 

 Bisciioff. (Ranke.) 



1, ovum with two segmentation spheres, the Zona pellucida heing surrounded by the Membrana gran- 

 ulosa ; 2, ovum with four segmentation spheres ; 3, ovum with eight segmentation spheres ; 4, ovum with 

 innumerable segmentation spheres, forming the mulberry mass. 



ment and form, different in different classes of animals, from which the 

 different tissues of the embryo are developed. 



The ova of animals are divided into two classes, — those in which the 

 entire yelk is concerned in the production of the embryo, and those in 

 which a part only serves for this purpose,— while the remainder of the cell- 

 contents is drawn upon for the nutritive needs of the embryo. The first 

 of these which undergoes total segmentation is termed a holobladic egg; 

 the second undergoes only partial segmentation, and is termed a mero- 

 blantic egg. The ovum of mammals serves for a type of the former 

 class; the ovum of birds is typical of the second class. As already 

 mentioned, the mammalian ovum represents a typical cell; the ovum of 

 the bird differs in many points. Beneath the yelk-membrane is a layer 

 of minute flattened cells, which gradually disappear during the maturing 

 of the egg ; while the yelk consists of two parts, one serving for the 

 development of the embryo, the other for its nutrition. The part from 



