28 GENERAL CONCEPTIONS. 



develop into a perfectly typical dorsal surface. The frog's 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 produces an absolutely 

 normal back and nervous system, normal as to form and function, though lack- 

 ing 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 experi- 

 ments by Oskar Schultze demonstrated that both the unpigmented and the 

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

 Another class of experiments, which were first made by Hans Driesch, have 

 revealed that the earliest cells (segmentation spheres, blastomeres, or cleavage 

 cells, as they are variously called) produced by the ovum preserve the undiffer- 

 entiated 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 normally 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 inver- 

 tebrate. Even more remarkable larvse 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 suc- 

 cessfully on the eggs of Clytia, and to have obtained one-sixteenth larvae. 



The facts offered suffice to illustrate the two aspects of our conception of the 

 undifferentiated condition of living matter. The first aspect is morphological 

 and presents to us the apparent uniformity of the visible minute structure of 

 protoplasm. While we readily admit that the uniformity may be only apparent 

 in the sense that we fail to observe fine differences, yet we none the less maintain 

 that the uniformity is real, because there is an absence of variations of structure 

 comparable to the variations which we can observe in the cells of adult tissues. 

 The second aspect is physiological, and offers to our view the wide range of possi- 

 bilities in the future developmental history and growth of the protoplasm. The 

 fate of the protoplasm of any given part of the ovum is not fixed ; but if its con- 

 ditions of development are changed, its fate is changed. A few years ago the 

 mosaic hypothesis was advanced by W. Roux, and it has been vigorously de- 

 fended by him. According to the mosaic theory, the egg is a mosaic pattern, 



