8o6 



DEVELOPMENT. 



[CH. LVIII. 



it by the spermatozoon. This is effected by the extrusion of the 

 two polar globules. The second polar globule is the one which 

 contains the germinal plasma, or the hereditary male element. 

 In those animals which can reproduce their species for many 

 generations without the intervention of a fresh male (partheno- 

 genesis), the second polar globule is not extruded. 



Fig. 617 represents a fertilized ovum, and if its appearance is 

 compared with that of the ovarian ovum (fig. 614), there is not 

 much anatomical distinction to be noticed between the two. The 

 fertilized ovum is rather smaller, as it has discharged the polar 

 globules and the perivitelline fluid, and its nucleus is composed of 

 male and female elements ; but great indeed is the physiological 

 difference between the two, for the fertilized ovum now is a new 

 individual, though in a very rudimentary condition. These 



processes all take 

 place in the Fallopian 

 tube of the mammal 

 as the ovum travels 

 towards the uterus, 

 and then the process 

 of segmentation or 

 cell division begins. 

 There is no doubt 

 that the important 

 hereditary substance 

 is contained in the 

 chromosomes of the 

 nucleus. Of these an 



equal number is contributed by each sex, and in the subsequent 

 cell division that occurs, the number of chromosomes in the 

 nucleus is therefore always an even number. 



Segmentation. 



The ovum first divides into two cells, then each of these into 

 two again, and so on ; until at last it consists of a mulberry-like 

 mass of little cells, all still enclosed within the zona pellucida. 

 The polar globules are soon lost to view. Cell division is always 

 accompanied by the usual karyokinetic changes in the nucleus. 

 On cutting a section through the embryo at this stage, it is found 

 to consist of a single layer of cells arranged around a central 

 cavity filled with fluid shed out from the cells. This is called 

 the blastula stage or the stage of the imilaminar (one layered) 

 blastoderm. The cells by mutual compression become columnar 

 in shape, and soon the cells are arranged two layers deep ; the 



Polar globules 



Fig. 617. The fertilized ovum, or blastosphere. 



