120 COMPARATIVE ANATOMY 



groups passes to opposite poles of the spindle. At the close 

 of division the two daughter-spermatocytes have given rise to 

 four spermatozoa, each of which contains two chromosomata — 

 i.e. half the characteristic number. Such a mitosis as this, 

 resulting in the halving of the number of chromosomata, is 

 known as a reducing division. 



In the ovum the process is essentially the same. The 

 primitive ova divide and sub-divide for some time with 

 normal mitoses. At length a stage is reached when each 

 cell, now called an oocyte, ceases to divide, and enters on 

 a period of growth, during which it becomes enormously 

 larger through the accumulation of food-yolk in its cytoplasm. 

 Then two tetrad groups are formed preparatory to a division. 

 The division takes place in the same manner as in the case 

 of the spermatocyte, so far as the behaviour of the tetrad 

 groups is concerned, but only a very minute portion of 

 cytoplasm containing two dyad groups is segmented off frorn 

 the ovum. This minute cell is called the first polar body. 

 It must be noticed that both it and the ovum contain two 

 dyad groups, just as the two daughter-spermatocytes contained 

 each two dyad groups. Both ovum and polar body im- 

 mediately prepare for a second division, and in each the 

 two dyad groups are halved in the process of mitosis. The 

 end result is the formation of (i) an ovum, containing two 

 chromosomata; (2) three polar bodies, one formed by the 

 second division of the ovum, the other two by the division 

 of the first polar body, each containing two chromosomata. 

 The polar bodies are not destined for further development. 

 They may persist for some time under the vitelline membrane, 

 but they eventually disintegrate and disappear. The ovum, 

 however, is ready for fertilisation. 



Fertilisation consists essentially in the union of the nucleus 

 of the spermatozoon (best called the sperm-nucleus) with that 

 of the ovum (best called the egg-nucleus). Each of these, 

 as the result of the reducing divisions, has half the number 

 of chromosomata characteristic of the species. By their 

 union the full number is restored, and the ovum is capable 

 of further division and development. 



The essential facts of fertilisation in Ascaris megalocephala 

 are as follows : — 



Whilst the ovum is dividing to form the second polar body, 



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