v ASCARIS 191 



is four. To make the statement complete we must say the diploid number 

 of chromosomes is in the male 4 + x and in the female 4 + 2x, x being the 

 inconspicuous sex chromosome. 



DIFFERENTIATION OF SOMA FROM GONAD 



Another series of phenomena of great general interest which have 

 been worked out more fully in Ascaris megalocephala than in any other 

 animal have to, do with the marking off of those cells which constitute 

 the soma from those of the gonad. In A. megalocephala this has been 

 found to occur at the earliest possible stage of development when the 

 zygote has divided into its first two blastomeres. As these commence 

 the next mitosis a difference becomes apparent between them. In one 

 the process is perfectly normal, the four chromosomes undergoing longi- 

 tudinal splitting precisely as in the first mitosis. In the other blastomere 

 however before the chromosome splits it undergoes transverse segmen- 

 tation. The swollen club-shaped ends of the chromosome drop off, 

 while the more slender central portion segments up into a number of 

 little pieces (Fig. 87, I, left-hand nucleus). It is only these small pieces 

 which undergo the splitting process and enter into the composition of 

 the nuclei of the two daughter cells. The club-shaped ends are simply 

 left lying in the cytoplasm, by which they are gradually digested and 

 destroyed. The result is that in the four-blastomere stage (Fig. 87, II) 

 two cells have nuclei with the full amount of chromatin while the 

 other two have nuclei with greatly diminished amount of chromatin. 

 With this diminution in the amount of chromatin the last-mentioned two 

 cells have become somatic cells. Throughout the numerous subsequent 

 mitoses during the course of development the descendants of these cells 

 remain with nuclei relatively poor in chromatin. 



When the two blastomeres (of the 4-cell stage) whose nuclei contain 

 the full amount of chromatin undergo mitosis one of the two repeats the 

 process of eliminating the club-shaped ends of its chromosomes, so that 

 two more somatic cells are produced, while the other cell divides normally 

 so that there are again in the 8-cell stage two cells with the full amount of 

 chromatin. 



Exactly how often this process of diminution of the chromatin is 

 repeated in normal cases whether three times as in the figure, or four 

 times or five times is not absolutely determined, but in any case the 

 end-result, when it has taken place for the last time, is that all the cell- 

 nuclei of the embryo have undergone, or in the case of one nucleus are 

 undergoing, the process of diminution so that one nucleus alone retains 



