ox PHYSIOLOGICAL MORPHOLOl: V. 



59 



5. The formation of the embryo begins with the formation 

 of the gastrula. The hollow sphere (the blastula, Fig. 26) 

 begins to fold at one place {a, c, b, Fig. 26), and an invagina- 

 tion takes place (Fig. 27). The formation of the fold {a, c, b, 

 FTg. 27) is due, undoubtedly, to unequal growth. The fact 

 that invagination, and not evagination (Fig. 28), takes place 



Fig. 



Fir,. 28. 



depends upon other conditions. So far as the first phenom- 

 enon is concerned, there are different possibilities by which 

 unequal growth may lead to a process of invagination. It is 

 possible, for instance, that the part c (Fig. 26), between a and 

 /;, may begin to grow more rapidly than the rest. But such 

 an unequal growth can only be the outcome of a chemical 

 difference between the parts a, c, b, and the rest. Such a 

 distribution of chemically different material e.xists in the 

 blastula which originates from the extra ovate, as well as in 

 the one which develops within the membrane. In the normal 

 ovum this distribution of material finds, perhaps, its first mor- 

 phological expression in the formation of micromeres. But 

 Driesch has shown that all possible variations in the formation 

 of micromeres may occur without affecting the formation of 

 the embryo. A necessary condition for the formation of the 

 embryo is the chemical differentiation which leads to unequal 

 growth; but whether this differentiation exists from the first, 

 or whether it takes place in the eight or the sixteen or the 

 sixty-four-cell stage is of minor importance. In order to 

 demonstrate this I made eggs burst in the two, four, sixteen, 

 and sixty-cell stages. In all cases except the sixty-cell stage 

 I produced the doublets, triplets, etc. The only condition 



