CELL CONJUNCTION

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Two gametes by uniting, will contribute two chromosomes each to the "zygote," the cell which is the offspring of their union, which will thus regain the original number of chromosomes, and all the cells which come from its division and subdivision through perhaps many thousand generations, will retain this number till gametes are again formed.

Two uniting gametes are not usually alike. The female is large and charged with much nourishment, but has no attraction sphere; while the male has only its moiety of chromosomes and an attraction sphere, the cell substance being reduced to a minimum. The male and female gametes in plants are found in the pollen grains and ovules or unfertilized seeds respectively, and in animals in spermatozoa and ova. Spermatozoa have temporary tails with which they swim (Fig. 3).

So much for the cell as a unit of life. We must now consider briefly the relation of the cell to the living individual. The living individual may consist of one cell or of many. If it consist of one cell the individual identity of the cell and of the animal are the same. Vorticella (Fig. 4) is a unicellular animal, whose parts are sufficiently described in the subjoined letterpress. When such a cell divides, the result is two separate and independent unicellular individuals. After many generations some of these animalcules are known to lose half their chromosomes and become "gametes," with perhaps the distinct forms of ovum and spermatozoon; two two such gametes unite, and

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from the union arises a new cell, the zygote," which starts a new host of animalcules all alike and all independent but all arising from the divisions and subdivisions of the original zygote cell.

The Hydra (Fig. 5) is a very simple multicellular animal. It is simply a bag with a hole in it, having tentacles which are hollow, the cavity of the animal extending into them as the cavity of a glove does into its fingers. The wall of this bag consists, as the figure shows, of two layers of cells of different size and texture. Those of the outer layer are sensitive and can change their shape in such a way that the hydra can move as a whole. By these movements it can suck water into its central cavity through the hole, or can eject it. When this water contains scraps of nourishment it is the cells of the inner layer that take it up and digest it, building part of it up into their own cell substance and passing some on to the cells of the outer layer. The cells of the hydra are a co-operative system dividing the functions of life between them. They are also components of a single individual identity.

Most of the animals that we know are far more complicated than a hydra, both in their form or morphology and in their physiology or functions, but in principle they are the same. They are an assembly of cells dividing between them the functions of the life of the single individual identity of which they are the components. How does such a group of cells come to be so assembled together and to have one individual identity in common?

Let us keep to the Hydra as an example. At certain times the cells of the outer layer thicken into two pairs of lumps on the sides of the body. The cells in these lumps undergo the special form of division by which they become "gametes." Those in one pair of swellings becoming ova, and those in the other spermatozoa. The swellings burst, and ova and spermatozoa are turned loose into the water. A spermatozoon meets an ovum and they unite to form a "zygote " as already described. Each such zygote is as much an independent monocellular individual as is the Vorticella. But when the cell divides, the daughter cells remain together with one individual identity between them, and this union continues through further divisions (such as are shown in Figure 6,) until a morula or blackberrylike bunch of cells is formed. This morula becomes a hollow ball of one layer and then the cells begin to differentiate, those on one side taking the character of lining cells; these become depressed into a cup, just as a deflated indiarubber ball can be made into a cup of two layers; by growth round the edges, this cup becomes a bag with a small hole in it. It is then in all essentials a hydra, for growth alone will suffice for its elongation and the throwing out of tentacles. Now in this perfected hydra all the cells have arisen by division and differentiation from a single cell, during which division they have stayed together with one common individual identity; and so it is with the more complex animals. They were each of them once unicellular and have become what they are by cell division 11-(2415)

and cell differentiation, much more complex but in principle the same as that of a hydra. Nay more, the early stages of all of the higher animals are the same as those of a hydra, all pass through the bag-with-a-hole-in-it or gastrula stage. It would be most interesting to follow them through their further stages, but it would be irrelevant and would hinder our purpose.

It is enough for us, and yet it is vital for us, to grasp, that every cell springs from an antecedent cell; that every living individual is, in its beginning, unicellular that the mode of motion which we call life has, so far, been seen only in cells: that matter which is inanimate, that is, whose mode of motion is other than that of life, takes on the living mode of motion only when it enters a cell and is there caught up into the living motion of the matter of that cell. Life is a mode of motion, but matter only takes on this mode by association with matter already moving in that mode. How did matter first take on this mode of motion? That is a question which no one has yet succeeded in answering. It is a question, however, which I ask you to keep before your minds, as it must be realized by all who would have any understanding of the principles of biological doctrine and the difficulty of the questions which biological inquiry has yet to solve. It is a question to which we shall have to return, for, surprising though it may seem, the possibility of there being a God or life for man in a world to come seems to some to hang on the answer

THEORIES OF EVOLUTION

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which must be given to this, as yet, unanswered question !!

We must now state the doctrine of evolution, and give the reasons for believing it to be true, and the obstacles and difficulties which stand in the way of believing it.

The doctrine is that all living forms, both vegetable and animal, including man, are descended from one elementary unicellular form of life, indeed, ideally, from one single cell, or as Pooh-Bah said in The Mikado, from a "primordial atomic globule," which is the ancestor of the Cedar of Lebanon, of the hyssop on the wall and of Solomon who wrote about them; of the beasts that die of anthrax, of the microbes which are the cause of anthrax and also of M. Pasteur who discovered the microbes. As an idea, it imagines that complex and specialized forms have arisen from the simpler and more generalized forms, by process of the hereditary preservation of variations. There are several accounts of how these variations arose. Lamarck suggested that the individual improved in those points of its structure which were necessary for its preservation, and that these improvements were handed on to subsequent generations. The giraffe, for instance, had to stretch its neck to browse on leaves, and so from generation to generation their necks became longer, the spotted giraffe once having had a short neck like the

' I must also ask my readers to remember that I am now engaged in giving an account of Orthodox Scientific doctrine, "without note or comment": discussion at this stage would be out of place.