IV. i INITIAL STRUCTURE OF THE GERM 257 



Ascaris, Boveri has pointed out that the extruded outer ends of 

 the chromosomes are very irregularly distributed to the daughter- 

 cells, whereas they should surely, if they are determinants, be 

 equally divided between the two. More than that, however, 

 Boveri has found evidence which shows that not only does the 

 diminished chromatin not determine the characters assumed by 

 the cytoplasm, but that on the contrary it is the cytoplasm 

 which decides which chromosomes shall be diminished, and 

 which remain intact. 



Boveri has observed certain cases of dispermy in Ascaris, 

 which are followed by simultaneous division of the ovum into 

 four, consequent, presumably, on a quadripolar mitosis which is 

 due, in turn, to the presence of an extra pair of centrosomes. 

 The total number of chromosomes present in such eggs is 3 n, 

 where n is the reduced number, since each spermatozoon intro- 

 duces n. This number becomes doubled by division, and the 

 number is then Gn or 12, since n in Ascari* megalocephala v. 

 livalens = 2. These twelve chromosomes have to be distributed 

 over the four cells into which the ovum divides : their distri- 

 bution is irregular. The next division is described, as being 

 tangential in two of the cells, at angles of 45 to the first 

 divisions in the other two: it leads to the formation of two 

 groups of four cells each, in each of which the cells are arranged 

 in the T shape characteristic of the normal four-celled stage. 

 The normal four-celled stage is reached by (1) an equatorial 

 division, (2) followed by a meridional division in the animal cell 

 (A B), and a latitudinal division in the vegetative cell, which 

 separates a cell S 2 , which like A and B is somatic, from a cell P 2 

 at the vegetative pole. The chromosomes in P 2 are intact, in 

 A, B, and S z they are diminished (Fig. 155). 



Boveri argues that if the diminution of the chromosomes were 

 an intrinsic property of the chromatin, there should always be 

 6 (3 n) chromosomes intact and the rest diminished, just as there 

 are always 4 (2 n) intact in the normal egg, no matter how the 

 chromosomes had become distributed over the first four cells and 

 their descendants. This, however, is not the case. Whole 

 chromosomes are found only in those cells one in each group 

 of four which correspond in position to the P 2 cell of the 



JENKINSON S 



