Ill DIFFERENTIATION 87 



are therefore, after the next division, two figures of T, each 

 of which continues to segment in every respect like a 

 whole egg. 



III. This type occurs frequently in centrifuged eggs. Of 

 the four cells one is large and yolkless (animal), and behaves 

 as S l ; while the remaining three are small and full of yolk, 

 and each develops as a P v 



The disperm eggs continue to segment, but give rise to 

 irregular cell masses incapable of development beyond the 

 gastrular stage, and that only in type I. Two primordial 

 germ-cells were found in these gastrulae. 



The number of chromosomes in these eggs is of course 3 n. 

 There are four centres between which spindles are developed in 

 various ways to form a quadripolar figure, the chromosomes 

 being irregularly distributed on the equators of these. There 

 they divide, and their halves are pulled in the ordinary way 

 to the spindle-poles. The four cells may and do receive, 

 therefore, different shares of the available 6 n, i, e. twelve chro- 

 mosomes. Now in the normal egg there are always 2 n 

 chromosomes that remain intact, in each cell in the germ- 

 track. If, therefore, the diminution were an intrinsic pro- 

 perty of certain chromosomes (namely, the somatic) while 

 conversely the intact chromosomes remained so by virtue of 

 some internal cause, then there should be in a dispermic 

 egg exactly six intact, and six diminished, chromosomes, 

 and these would be irregularly distributed over the four cells 

 and their descendants. The reverse of this is the case. In 

 certain cells, the number of which differs in the three types, 

 the chromosomes remain entire, whatever their number may 

 be (it may be any number from two to twelve), while in the 

 other cells the chromosomes are diminished, whatever their 

 number. We know further that the diminution occurs in 

 the dispermic, as in the normal egg, in the cells of the animal 

 region, while in the vegetative cell or cells it is absent. It 

 is necessary therefore to suppose that it is some difference 

 in the cytoplasm of the animal hemisphere which causes the 

 diminution. 



We shall see in the next section that while there is evidence 



