CHAPTER XII 

 THEORIES OF DEVELOPMENT 



THE experimental work that Pfluger carried out in 1883 on the 

 effect of gravity on the cleavage of the frog's egg, and the con- 

 clusions that he drew from his experiments, mark the starting- 

 point for the modern study of experimental embryology. 1 We can 

 trace the influence of Pfluger's results through most of the more 

 recent work, and one of the conclusions reached by Pfluger, namely, 

 that the material of the egg may be divided by the cleavage planes 

 in any way whatsoever without thereby altering the position of the 

 embryo on the egg, is, I think, one of the most important results that 

 has yet been reached in connection with the experimental work on 

 the egg. Pfluger's analysis of the factors that direct the develop- 

 ment has also an important bearing on the interpretation of the 

 development of a whole embryo from a part of an egg. 



Pfluger found that in whatever position the frog's egg is turned 

 before it begins to divide, the first two planes come in vertically, and 

 the third horizontally, and that later the smallest cells are always 

 formed in the upper hemisphere. He concluded, therefore, that 

 gravity has some sort of influence in determining the position of the 

 planes of cleavage. Pfluger observed that the position of the median 

 plane of the body of embryos that have developed from eggs turned 

 into unusual positions does not, as a rule, correspond to the plane of 

 the first cleavage, but that the embryo generally lies on that meridian 

 of the egg that passes through the primary egg axis and the highest 

 point of the egg in its new position. Since any meridian may happen 

 to be placed uppermost, the embryo may, therefore, develop upon 

 any one of the primary meridians, and hence the material must be 

 isotropous around the primary axis. Furthermore, since the embryo 

 appears always below the middle of the egg, in whatever position 

 the egg may lie, we must conclude that in each meridian the material 

 is also isotropic. 



It may be pointed out that while more recent work has substanti- 

 ated, on the whole, the latter conclusions 2 of Pfluger, just stated, still 



1 These experiments have been quite fully described in my book on The Development of 

 the Frog's Egg. 



2 Not, however, the supposed action of gravity on the egg. 



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