V ASCARIS 1 8s 



into two. It will be noted (i) that each of the daughter cells contains 

 two, i.e. the haploid, number of chromosomes and (2) that each individual 

 chromosome is a dyad. 



Each of the two cells repeats the process of mitosis. A spindle is 

 again developed, having at its poles centrosomes arising by the division 

 of the original centrosome, and the two dyad chromosomes arrange them- 

 selves at its equator (Fig. 84, C, lower half), and become split into their 

 two constituent halves which move apart towards the poles of the spindle 

 as monad chromosomes. The cell-body becomes as before constricted 

 into two cells — each of which again contains the haploid number of 

 chromosomes, these being now monad or single in their nature, together 

 with a centrosome (Fig. 84, C, upper half). 



Thus the original cell from which we started is now represented by 

 four cells. Each of these cells (spermatids) gradually takes on the form 

 of a functional microgamete or spermatozoon. This (Fig. 86, 4) is quite 

 unlike the spermatozoa of most animals in appearance, being somewhat 

 conical in shape with a rather expanded base of soft protoplasm, by the 

 amoeboid movement of which the spermatozoon creeps. Within this 

 lie the two chromosomes and the centrosome, while the apical portion 

 is filled by a clear glassy body of unknown function. 



Female (Fig. 84, right-hand figures) 



In the ovary just as in the testis there is a special level at which 

 the mitotic nuclei show chromosomes tetrad in structure and haploid in 

 number. The cells are larger owing to the cytoplasm being distended 

 by large granules of reserve food-material or yolk. As mitosis com- 

 mences a spindle makes its appearance as in the male with the 

 two tetrad chromosomes at its equator, but this spindle is devoid 

 of centrosomes and it is situated close under the surface of the cell, 

 with its axis in a radial direction, i.e. perpendicular to the surface 

 (Fig. 84, A). Each tetrad splits apart into two dyads and then the 

 cell divides but instead of the two daughter cells being of approxi- 

 mately equal size, one of them is reduced to the smallest dimensions, 

 consisting of hardly more cytoplasm than is just sufficient to contain 

 the two dyads. This tiny cell is known as the first polar body (Fig. 

 84, B, I). 



The mitotic process is repeated in the large cell. Each dyad becomes 

 rotated through a right angle so that it takes up a radial position : its 

 constituent monads move apart towards the poles of the spindle. 

 The cell as a whole divides, and again one of the two daughter cells 



