Conklin has established the existence of a definite structure in the unfertilized eggs of Ascidians, Amphioxus, and many molluscs. In all cases the results of the isola­tion of the first blasto­meres seem to agree with the demonstrable structure of the unfertilized egg.

5. These examples may suffice to show that the egg has from the beginning a simple structure, and we will now point out by which means further differentia­tion may come about. Sachs suggested that all differentia­tion and the forma­tion of every organ presupposes the previous existence of specific substances responsible for the forma­tion. These substances which are now called internal secre­tions or hormones develop gradually during embryonic development. What exists first is a jelly-like block of protoplasmic material with a varying degree of viscosity and with just enough differentia­tion to indicate head and tail end, a right and left, and a dorsal and ventral side of the future embryo.

Aside from such simple differences phenomena of protoplasmic streaming contribute to the further differentia­tion. Such streaming begins, according to Conklin,[133] in the egg just before fertiliza­tion when the surface layer of the egg protoplasm

streams to the point of entrance of the sperm, and these movements may lead to the segrega­tion of different kinds of plasma in different parts of the egg and to the unequal distribu­tion of these substances in different regions of the egg.

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]

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]

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.”