12 GENERAL CONCEPTIONS. 



The uneven distribution of the yolk granules in the ova of mammals (compare 

 page 34) indicates that there are unlike regions, the morphological significance 

 of which, however, is not known yet. 



Two views as to the constitution of the ovum in relation to the structures 

 which arise from it have been brought forward. According to one view, each 

 part of the ovum is predestined to form a definite part of the adult and cannot 

 form any other part. According to the other view, the ovum is homogeneous 

 in its essential lack of true differentiation, and any part of it may form any part 

 of the adult if given the requisite opportunity. The first view is known as the 

 mosaic Jheory, the egg being compared to a mosaic, and was founded by Wilhelm 

 Roux (1888). The second view is known as the theory of isotropism and was 

 founded by Ernst Pfliiger in 1884. 



Pfliiger's theory of isotropism was based upon- his experiments on frogs' eggs. 

 Each egg has a small white area which normally, lies underneath, the larger, 

 darkly pigmented area of the egg alone showing from above. Out of the dark 

 area the back, with the nervous system and other parts, takes its origin. If the 

 eggs, freshly fertilized, are fastened with the white side up, then the white side pro- 

 duces an absolutely normal back and nervous system, normal as to form and func- 

 tion, though lacking the typical pigmentation. These observations were confirmed 

 by Born, who further discovered that the segmentation nucleus always rises 

 toward the upper side of the egg, and that the position of the nucleus determines 

 which part of the ovum shall become the dorsal side of the embryo. Another set of 

 experiments by Oskar Schultze demonstrated that both the unpigmented and the 

 pigmented sides of the same egg could be made to produce dorsal structures. An- 

 other class of experiments, which were first made by Hans Driesch, has revealed 

 that the earliest cells (segmentation spheres, blastomeres, or cleavage cells, as they 

 are variously called) produced by the ovum preserve the undifferentiated qualities 

 of the parent egg, and may develop in one way or another according to circumstances. 

 The egg of a sea-urchin divides into two cells, each of which multiplies and nor- 

 mally gives rise to half of the body of the animal. By somewhat violent shaking the 

 two cells may be artificially separated; each cell may then develop into a complete 

 larval sea-urchin, but of half the normal size only. Similar experiments have since 

 been made by several investigators, who have obtained like results with other 

 animals, vertebrate as well as invertebrate. Even more remarkable larvae have 

 been raised from blastomeres of the four-cell and eight-cell stages of segmentation, 

 producing larvae of one-fourth and one-eighth the normal size. Zoja claims to have 

 repeated the experiment successfully on the eggs of Clytia and to have obtained 

 one-sixteenth larvae. 



Roux's mosaic theory was based on W. His's principle of the organ-forming 

 areas of the germ, or, as it has been also termed, the law of germinal localization. 

 His pointed out that in the normal course of development every spot in the 

 blastoderm corresponds to some future organ, and suggested that logically it is 



