Conklin says[44] that in the ascidian Styela "there are four or five substances in the egg which differ in color, so that their distribution to different regions of the egg and to different cleavage cells may be easily followed, and even photographed, while in the living condition. The peripheral layer of protoplasm is yellow and it gathers at the lower pole of the egg, where the sperm enters, forming a yellow cap. This yellow substance then moves, following the sperm nucleus, up to the equator of the egg on the posterior side, and there forms a yellow crescent extending around the posterior side of the egg. On the anterior side of the egg a gray crescent is formed in a somewhat similar manner, and at the lower pole between these two crescents is a slate-blue substance, while at the upper pole is an area of colorless protoplasm. The yellow crescent goes into cleavage cells which become muscle and mesoderm, the gray crescent into cells which become nervous system and notochord, the slate-blue substance into endoderm cells, and the colorless substance into ectoderm cells. Thus within a few minutes after the fertilization of the egg, and before or immediately after the first cleavage, the anterior and posterior, dorsal and ventral, right and left poles are clearly distinguishable, and the substances which will give rise to ectoderm, endoderm, mesoderm, muscles, notochord, and nervous system are plainly visible in their characteristic positions." Conklin followed these cells in every division until the embryo was developed, making a complete genealogy up to the ovum proper. [44] _Heredity and Environment_, p. 123, Oxford Press. De Vries[45] assumed that the character of each cell is determined by "Pangens" that migrate from the nucleus into the protoplasm. Driesch and Oscar Hertwig held that the peculiar development of a given blastomere is a result of its relation to the remainder of the cell-mass, an outcome of the action upon it by the whole system of cells of which it is a part. Hertwig said:[46] "Each of the first two blastomeres contains the formative and differentiating forces not simply for the production of a half-body, but for the entire organism; the left blastomere develops into the left half of the body only because it is placed in relation to a right blastomere." Wilson[47] and Driesch[48] came to the same conclusion about the time Hertwig wrote. Driesch said:[49] "The relative position of a blastomere in the whole determines in general what develops from it; if its position be changed it gives rise to something different; in other words, its prospective value is a function of its position." [45] _Intracelluläre Pangenesis._ Jena, 1889. [46] _Jenaische Zeitschrift_, 1892, 1. [47] _Journal of Morphology_, 1893, 1894. [48] _Studien_, iv, p. 25. [49] _Ibid._, p. 39.