22 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



does not prevent it from being classed with other irritable forms of living cell- 

 substance as protoplasm. In spite of differences in structure and composition, 

 nerve protoplasm and muscle protoplasm are found to have many points of 

 resemblance. An explanation of the physiological resemblances may be found 

 in their common ancestry. All the cells of the many structures of the animal 

 body are descended from the two parent cells from which the animal isdeveloped. 

 The fertilized ovum divides, and two cells are formed, these new cells divide, 

 and so the process continues, the developing cells through unknown causes be- 

 coming arranged to form more or less definite layers and groups, which by means 

 of foldings and unequal growths develop into the various structures and organs 

 of the fetus. At the same time that the division is going on, the total amount 

 of material is increasing. Each of the cells absorbs and assimilates dead food- 

 material, and this dead material is built into living substance. During this 

 process of development and growth the cells of special tissues and organs 

 acquire special anatomical and chemical characters. This development of 

 specialized cells is termed cell-differentiation. Hand in hand with the ana- 

 tomical and chemical differentiation goes a physiological differentiation. The 

 protoplasm of each type of cell, while retaining the general characteristics of 

 protoplasm, has certain physiological properties developed to a marked degree 

 and other properties but little developed, or altogether lacking. The fertilized 

 ovum does not have all the anatomical and chemical characteristics of all the 

 cells which are descended from it, not at least in just the form in which they 

 are possessed by these cells, and it cannot be assumed that its living sub- 

 stance possesses all the physiological properties which are owned by its 

 descendants. Many of these properties it must have, for many of them are 

 essential to the continuance of life of all active cells, — such as the power to take 

 in, alter, and utilize materials which are suitable for the building up and repair 

 of the cell-substance, the power of chemically changing materials possessing 

 potential energy so that the form of actual energy which is essential to the per- 

 formance of the work of the cell shall be liberated, and the power to give 

 off the waste materials which result from chemical changes. The protoplasm 

 of the ovum, to have these powers, has properties closely allied to absorption, 

 digestion, assimilation, respiration, excretion ; and, in consideration of the special 

 function of the ovum, we may add that it possesses the property of reproduc- 

 tion. The question of its possessing the characteristic properties of muscle and 

 nerve protopla-m cannot be answered off-hand. Careful study, however, has 

 shown the ovum of Hydra to possess irritability, conductivity, and contractility. 

 It undergoes amoeboid movements, as was first shown by Kleinenberg. 

 Balfour, 1 in writing of the development of the ova of Tubularidae, which is 

 of a type similar to Hydra, says: " The mode of nutrition of the ovum may 

 be very instructively studied in this type. The process is one of actual feed- 

 in-, much a- an amoeba might feed on other organisms." Something similar 

 seems to be true of the ova of echinodermata. During impregnation various 

 movements are described implying the properties of irritability, conductivity, 

 and contractility. Thus in the case of Asterias glacialis, when the head of the 

 1 Comparative Embryology, pp. 17, 29. 



