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Conklin had found out in 1904 and 1905 that the egg cytoplasm in Ascidians is not only composed of different materials, but that these give rise to definite structures in the embryo later on. If we look at these views from one angle, the apparent conflict disappears, as Professor Conklin points out. We can still presume that all the factors of inheritance are carried in the nucleus.
It is well known that the protoplasm of the cell effects various movements within its envelope; on the other hand, indirect cell-division is the outcome of very complex operations, some involving the nucleus and others the cytoplasm. These latter commence by the doubling of the centrosome, a small spherical body alongside the nucleus.
Food particles and waste. 5. Cell-wall. 6. Masses of active material found in certain cells, called plastids. Surrounding the nucleus is the main body of the cell, sometimes referred to as the "protoplasm." Since the protoplasm forms all parts of the cell, this substance is more properly called the cytoplasm, or cell plasm.
The nucleus, and especially the chromosomes, are supposed in some unknown way to influence or govern the metabolism of the cytoplasm. From this point of view the hypothesis mentioned above that the sex-difference in the gametes is not qualitative but quantitative is probably nearer to the truth.
The earliest process of inorganic differentiation in the structureless body of the Monera led to its division into two different substances the caryoplasm and the cytoplasm. The caryoplasm is the inner and firmer part of the cell, the substance of the nucleus. The cytoplasm is the outer and softer part, the substance of the body of the cell.
Surrounding and inclosing the cytoplasm, in many cells, is a thin outer layer, or membrane, which affords more or less protection to the contents of the cell. This is usually referred to as the cell-wall. A fourth part of the cell is also described, being called the attraction sphere. This is a small body lying near the nucleus and coöperating with that body in the formation of new cells.
The nucleus segmented and migrated into that part of the superficial cytoplasm which remained alive, and this proceeded to develop that particular part of the embryo to which it would have given rise if the rest of the egg had not been killed. There was no regeneration of the part killed, no formation of a complete embryo. It may be pointed out that segmentation in the insect egg is peculiar.
If one of these in reduction expelled a Y chromosome, the other an X, then one would retain an X and the other a Y. Each was fertilised by a sperm, one becoming therefore XX or male and the other XY or female. It may be supposed that as there was only the cytoplasm of one ovum, each nucleus would determine the characters of half the individual developed.
The nuclei multiplied by segmentation migrate into the superficial cytoplasm surrounding the yolk, and then this cytoplasm segments, and each part of the cytoplasm develops into a particular region of the embryo.
Food particles, wastes, and other substances may also be present in the cytoplasm. *Importance of the Cells.*—The cells must be regarded as the living, working parts of the body. They are the active agents in all of the tissues, enabling them to serve their various purposes. Working through the tissues, they build up the body and carry on its different activities.
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