GENERATION AND DEVELOPMENT 701 



philosopher or idiot, or a humble intestinal worm, are in all cases 

 identical. In the development of the ovum the stages through 

 which the higher animals have passed before they became higher 

 may practically be witnessed or readily visualised. There is nothing 

 in the early development of the ovum which sharply defines man 

 formation from dog, cat, or pig formation ; each goes through the 

 same phases up to a certain point, and then comes the parting of 

 the ways. The unity of type of the embryo in its early stages is 

 succeeded by its sharp differentiation ; this is even more remarkable 

 than the incomprehensible starting-point. 



It is impossible in a work of this kind to enter more deeply into 

 the physiology of the cell, or to relate the little which is known 

 regarding it. It is obvious that a knowledge of the cell is intimately 

 connected with the nature of life. All the physiological processes 

 of the body considered in previous chapters are carried out by the 

 cells, and the methods by which they work are chemical, physical, 

 and biological problems of extraordinary difficulty. 



Maturation of the Ovum. — The maturation of the ovum is 

 not the same as its fertilisation. Maturation is concerned 

 with the production of a perfect from an immature ovum, of a 

 mature from an embryonic cell. This process is effected on the 

 lines above indicated. It has been stated that the cells in the 

 ovary at birth are imperfect ; their number is laid down, and this 

 we have seen amounts to several thousands. They are com- 

 pleted as required, and are only matured some months or years 

 after birth, depending on the species of animal. It is obvious 

 that only a few are matured at one time, and, again, the species 

 of animal determines whether they are matured, as in the woman, 

 at short regular intervals or at relatively long intervals. The 

 periodical development of a mature egg or eggs constitutes the 

 essential feature of maturation. 



The primitive ovum takes its first step towards maturation 

 by the process of cell division. Two cells unequal in size result : 

 one is still the ovum, the other the first polar body. The number 

 of chromosomes in the immature ovum was originally, in the 

 case of the mare, twenty-six ; as the result of the formation of 

 the first polar body, these twenty-six fuse and form thirteen. 

 Before the first polar body is cut adrift the thirteen chromosomes 

 split, so that thirteen go to the polar body and thirteen remain 

 with the ovum. Though these two cells contain only half the 

 number of chromosomes contained by a somatic cell, yet each 

 is a whole chromosome, and not half a one, as in the division of 

 somatic cells. The ovum next divides a second time, and 

 extrudes a second polar body. This time the thirteen chromo- 

 somes either split or divide, thus forming twenty-six ; half pass 

 to the second polar body, and half remain with the parent cell, 

 the ovum. So that the formation of the two polar bodies has 

 caused the ovum to suffer a loss in the number of chromosomes, 



