THE CATALYST ENTELECIIY IN DIFFERENTIATION 185 



Though the first few cleavages of the zygote do not greatly 

 increase the size of the cell group, differentiation is already mani- 

 fest at the 16-cell stage, distinction being evident between the 

 part which later implants the egg on the maternal tissue, and the 

 trophoblasts, the cells later to form the fetal membrane, which 

 divide more rapidly and therefore are smaller than the other cells. 

 Next is seen the formation of the blastocyst, a vesicle which cups 

 into a hollow sphere that distends with fluids possibly secreted by 

 the trophoblasts; and now the cell mass visibly enlarges. 



The outstanding feature of blastocyst development is the ab- 

 sorption of tremendous quantities of water. C. B. Davenport 5 

 estimated that after six weeks the human egg weighs about a 

 gram, nearly 500,000 times its initial weight; and 98 per cent of 

 the weight increase is water. The blastocyst develops the embry- 

 onic envelope and the placenta, the latter really special tissue 

 formed where the circulatory systems of embryo and mother come 

 into contact. The development of the placenta in mammals has 

 been shown by Pincus to be controlled by progesterone, the corpus 

 luteum hormone. 6 



Following attachment of the egg to the placenta, formation of 

 the embryo continues by segmentation and differentiation of the 

 inner cells; and the amnion and yolk-sac vesicles develop and 

 flatten together to form, with an intervening layer of cells (meso- 

 blast), the triple-layered germ disc which comprises the embryo. 

 In man this occurs about the third week, and by the end of the 

 second month the embryo (in the case of man) is recognizable as 

 human. Eyes, mouth region, and limb buds may be seen by the 

 fourth month and the organs are formed the month before. The 

 "ground plan," as Brody puts it, "is laid very early in life and 

 rounded out later through enlargement and remodeling of the 

 parts. Age changes in shape are due to differences in growth 

 rates of the constituent parts. There is an orderly sequence or 

 gradient 1 in organ formation. The head has precedence in de- 

 velopment over the tail end, and so on in cephalochordal sequence 

 for the other organs. The sequences may be associated with 

 organizer and hormone action." 



How is all this orderly development to be explained? The 

 view suggested by W. Roux in 1883 that mitotic division of the 

 zygote establishes development by a qualitative distribution of 

 the nuclear germ plasm was a great advance beyond the medieval 

 notion that a homoculus or "little man" existed initially in the 



