The Story of the Living Machine / A Review of the Conclusions of Modern Biology in Regard / to the Mechanism Which Controls the Phenomena of Living / Activity

by H. W. Conn

This factor of heredity again we must trace back to the machinery of the cell. We have seen in the previous pages evidence for the wonderful nature of the chromosomes of the cells. We can not pretend to understand them, but they must be extraordinarily complex. We have seen proof that these chromosomes are probably the physical basis of heredity, since they are the only parts of each parent which are handed down to subsequent generations. With these various facts of cell division and cell fertilization in mind, we can reach a very simple explanation of fundamental features of heredity. The following is an outline of the most widely accepted view of the hereditary process.

Recognizing that the chromosomes are the physical basis of hereditary transmission, we can picture to ourselves the transmission of hereditary characters something as follows: As we have seen, the fertilized egg contains an equal number of chromosomes from each parent (Fig. 42). Now when this fertilized cell divides, each of the rods splits lengthwise, half of each entering each of the two cells arising from the cell division. From this method of division of the chromosomes it follows that the daughter cells would be equivalent to each other and equivalent also to the undivided egg. If the original chromosomes contained potentially all the hereditary traits handed down from parent to child, the chromosomes of each daughter cell will contain similar hereditary traits. If, therefore, the original fertilized egg possessed the power of developing into an adult like the parent, each of the daughter cells should likewise possess the power of developing into a similar adult. And thus each cell which arises as the result of such division should possess similar characters so long as this method of division continues. But after a little in the development of the egg a differentiation among the daughter cells arises. They begin to acquire different shapes and different functions. This we can only believe to be the result of a differentiation in their chromatin material. In the cell division the chromosomes no longer split into equivalent halves, but some characters are portioned off to some cells and others to other cells. Those cells which are to carry on digestive functions when they are formed receive chromatin material which especially controls them in the performance of this digestive function, while those which are to produce sensory organs receive a different portion of the chromatin material. Thus the adult individual is built up as the cells receive different portions of this hereditary substance contained in the original chromosomes. The original chromosomes contained all hereditary characters, but as development proceeds these are gradually portioned out among the daughter cells until the adult is formed.

From this method of division it will be seen that each cell of the adult does not contain all the characters concealed in the original chromosomes of the egg, although each contains a part which may have been derived from each parent. It is thought, however, that a part of the original chromatin material does not thus become differentiated, but remains entirely unchanged as the individual is developing. This chromatin material may increase in amount by assimilation, but it remains unchanged during the entire growth of the individual. It thus follows that the adult will contain, along with its differentiated material, a certain amount of the original physical basis of heredity which still retains its original powers. This undifferentiated chromatin material originally possessed powers of producing a new individual, and of course it still possesses these powers, since it has remained dormant without alteration. Further, it will follow that if this dormant undifferentiated chromatin should start into activity and produce a new individual, the new individual thus produced would be identical in all characters with the one which actually did develop from the egg, since both individuals would have come from a bit of the same chromatin. The child would be like the parent. This would be true no matter how much this undifferentiated material should increase in amount by assimilation, so long as it remained unaltered in character, and it hence follows that every individual carries around a certain amount of undifferentiated chromatin material in all respects identical with that from which he developed.

Now whether this undifferentiated germ plasm, as we will now call it, is distributed all over the body, or is collected at certain points, is immaterial to our purpose. It is certain that portions of it find their way into the reproductive organs of the animal or plant. Thus we see that part of the chromatin material in the egg of the first generation develops into the second generation, while another part of it remains dormant in that second generation, eventually becoming the chromatin of its eggs and spermatozoa. Thus each egg of the second generation receives chromosomes which have come directly from the first generation, and thus it will follow that each of these eggs will have identical properties with the egg of the first generation. Hence if one of these new eggs develops into an adult it will produce an adult exactly like the second generation, since it contains chromosomes which are absolutely identical with those from which the second generation sprung. There is thus no difficulty in understanding why the second generation will be like the first, and since the process is simply repeated again in the next reproduction, the third generation will be like the second, and so on, generation after generation. A study of the accompanying diagram will make this clear.

In other words, we have here a simple understanding of at least some of the features of heredity. This explanation is that some of the chromatin material or germ plasm is handed down from one generation to another, and is stored temporarily in the nucleii of the reproductive cells. During the life of the individual this germ plasm is capable of increasing in amount without changing its nature, and it thus continues to grow and is handed down from generation to generation, always endowed with the power of developing into a new individual under proper conditions, and of course when it does thus give rise to new individuals they will all be alike. We can thus easily understand why a child is like its parent. It is not because the child can inherit directly from its parent, but rather because both child and parent have come from the unfolding of two bits of the same germ plasm. This fact of the transmission of the hereditary substance from generation to generation is known as the theory of the continuity of germ plasm.

Such appears to be, at least in part, the machinery of heredity. This understanding makes the germ substance perpetual and continuous, and explains why successive generations are alike. It does not explain, indeed, why an individual inherits from its parents, but why it is like its parents. While biologists are still in dispute over many problems connected with heredity, all are agreed to-day that this principle of the continuity of the heredity substance must be the basis of all attempts to understand the machinery of heredity. But plainly this whole process is a function of the cell machinery. While, therefore, the idea of the continuity of germ substance greatly simplifies our problem, we must acknowledge that once more we are thrown back upon the mysteries of the cell. Until we can more fully explain the cell machine we must recognize our inability to solve the fundamental question of why an individual is like its parents.

FIG. 50.—Diagram illustrating
the principle of heredity.

A represents an egg of a starfish. From one half, the unshaded portion, develops the starfish of the next generation, B. The other is distributed without change in the ovaries, ov, of the individual, B. From these ovaries arises the next egg, A', with its germ plasm. This germ plasm is evidently identical with that in A, since it is merely a bit of the same handed down through the individual, B. In the development of the next generation the process is repeated, and hence B' will be like B, and the third generation of eggs identical with the first and second. The undifferentiated part of the germ plasm is thus simply handed on from one generation to the next.]