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.
[136] Driesch, H., _Science and Philosophy of the Organism_, i., p. 104.
[137] Herbst, C., _Formative Reize in der tierischen Ontogenese_. Leipzig, 1901.
6. As a consequence of further changes definite anlagen or buds originate later in the embryo which are destined to give rise to definite organs. Thus in the tadpole early mesenchyme cells are formed which are the anlagen for the four legs, which will grow out under the proper conditions. These anlagen are specific inasmuch as from the anlage of a foreleg only a foreleg, and from the anlage for a hindleg only a hindleg, will develop. Braus[138] has proved this by transplanting the anlage of a foreleg to different parts of the body. No matter into which part of the body they are transplanted the mesenchyme cells for the foreleg will give rise to a foreleg only; even if they are transplanted into the spot from which the hindlegs grow out under natural conditions. There is therefore nothing to indicate "regulation."