12 INTRODUCTORY I 



whole row then undergoes longitudinal fission, and the 

 daughter rows pass to the opposite centrosomes in the ana- 

 phase of the mitosis, to form the daughter nuclei. The 

 swollen ends are however cast out into the cytoplasm, quite 

 irregularly, passing sometimes into both, sometimes into one 

 only of the blastomeres A and B, and disintegrate and dis- 

 appear. The cell in which the chromosomes are thus 

 diminished is purely somatic ; it gives rise to the ectoderm 

 of the embryo. In the other cell, P lt the chromosomes 

 remain intact. This cell is the parent not merely of certain 

 somatic cells, but also of the future germ-cells, and we shall 

 see that in the course of segmentation each cell which con- 

 tains in itself the material for the future germ-cells, or lies 

 in the germ-track, retains intact chromosomes, while in every 

 cell that is destined to give rise only to some part of the soma, 

 the chromosomes undergo diminution. The cell E M St is such 

 a cell ; it contains the material for the endoderm, some of the 

 mesoderm, and the stomodaeum, while from its sister cell, 

 P 2 , the gonad, as well as some mesoderm, is to be derived. 



The P 2 cell now shifts round to one side (the future posterior 

 end), so that the four cells lie in a rhombus. They prepare 

 for the next division. A and B are divided lengthways into 

 right and left moieties, the chromosomes appearing in the 

 ' diminished ' form as granules. 



E M St (S 2 ) divides transversely into an anterior cell, M St 

 (mesoderm and stomodaeum), and a posterior cell, E (endo- 

 derm). In the mitosis, diminution of the chromosomes occurs. 

 P 2 divides with intact chromosomes into a lower anterior 

 cell, Po, and an upper posterior cell, C (or S 3 ). In this cell 

 diminution will occur prior to its division. It gives rise to 

 mesoderm. 



By further divisions the number of ectoderm cells is 

 increased (derivatives of AB), and of endoderm cells likewise, 

 the mesoderm (M) is separated from the stomodaeum (St) 

 material, and P 3 divides into P 4 and D (or $ 4 ). The latter is 

 somatic (mesoderm), and diminution of the chromosomes occurs 

 in it. P 4 is now purely germinal, and the chromosomes remain 

 intact in all its descendants. 



