[133] Conklin, E. G., _Heredity and Environment in the Development of Man_. Princeton University Press, 1915. The reader is referred to this book for the literature and main facts on the structure of the egg; it should also be stated that Conklin’s book is one of the best introductions to modern biology in the English literature. One of the most striking cases of this is found in the Ascidian Styela in which there are four or five different kinds of substances in the egg which differ in colour, 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 when it gathers at the lower pole of the egg where the sperm enters it forms 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 just below the equator. On the anterior side of the egg a grey 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 colourless protoplasm. The yellow crescent goes into cleavage cells which become muscle and mesoderm, the grey crescent into cells which become nervous system and notochord, the slate-blue substance into endoderm cells, and the colourless 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.[134] [134] Conklin, E. G., _loc. cit._, p. 117. We may finally allude briefly to the fact that when once a number of tissues are differentiated each one may influence the other by calling forth tropistic reactions. Thus the writer showed that in the yolk sac of the fish _Fundulus_ the pigment cells lie at first without any definite order but that they gradually are compelled to creep entirely on the blood-vessels and form a sheath around them with the result that the yolk sac assumes a tiger-like marking.[135] Driesch[136] has pointed out that the mesenchyme cells are directed in their migration; and it seems that the direction of the growth of the axis cylinder is determined by the tissues into which it grows. The idea of tropistic reactions in the formation of organs has been discussed by Herbst.[137] [135] Loeb, J., _Jour. Morphol._, 1893, xiii., 161; _The Mechanistic Conception of Life_. Chicago, 1912, p. 106.