IV. i INITIAL STRUCTURE OF THE GERM 255 



FIG. 155. The process of chromatin diminution as seen in the 

 somatic cells of Ascaris megalocephala. (After Boveri, 1899.) 



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

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

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



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

 tin are seen. S 2 (E M St), second soinatoblast 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 EM St the chromatin is being diminished. Division of P 2 into 

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

 n, 3, primary ectoderm of left side. 



5. The endoderm (E lt 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 (J>). 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. 



