GROWTH OF THE GERM-CELLS 



13 



S 2 (EMSt) 



FIG. 5. The process of chromatin diminution as seen in the somatic 

 cells of Ascaris megcUocephaJa. (After Boveri, 1899.) 



1. Mitosis in the 2-celled stage. In the first somatic cell (5 X or AB), 

 the primary ectoderm, the chromatin undergoes diminution, not in the 

 germ-cell (Pj). la, chromosomes being 'diminished '. 



2. 4-cell stage, T-shape. In A and B the discarded masses of chroma- 

 tin are seen. S z (EMSt}, second somatoblast or endomesostomodaeal 

 rudiment. 



3. 4-cell stage, lozenge-shape. In A and B the next mitosis is begin- 

 ning, in P 2 and E M St the nuclei are in the resting stage. A is anterior, 

 A and B are dorsal, all four cells lie in one plane, the sagittal plane of 

 the embryo. 



4. In E M St the chromatin is being diminished. Division of P 2 into 

 P 3 and S 5 (C), the secondary ectoderm, a, b, primary ectoderm of right, 

 a, 3, primary ectoderm of left side. 



5. The endoderm (E l ,E 2 ) has now been separated from mesoderm and 

 stomodaeum. P 3 has just divided into P 4 and S 4 (d , tertiary ectoderm. 



6. Diminution of chromatin in S 4 (Z>). The four endoderm cells (E) 

 beginning to be invaginated : on each side two mesoderm cells (M ) in 

 which granular chromosomes may be seen, and two stomodaeal cells (St). 

 Ventral view. 



