262 EVOLUTION AND ANIMAL LIFE 



some with a small amount of cytoplasm modified primarily into 

 an organ of locomotion, the tail. A physiological division of 

 labor is here met with which admirably meets two diametrically 

 opposed requirements. The one of these demands that the 

 conjugating cells be highly motile, and consequently small, 

 in order that they may be able to come together in the water 

 in which they are usually set free. The second requires that 

 there be furnished a sufficient amount of nutritive material 

 for the nourishment of the embryo until it arrives at a stage of 

 growth in which it can shift for itself. These two necessities 

 have been met by the physiological division of labor between 

 the two conjugating cells. The one, the sperm cell, has become 

 reduced in size with a corresponding gain in motility, the 

 other, the egg cell, has had food yolk stored up in it, and its 

 consequent increased size prevents any more than a very slight 

 degree of independent movement, if any. Different stages of 

 these modifications may be met with among unicellular forms, 

 as illustrated in Pandorina, Eudorina, and Volvox, to which 

 might be added many others. In Pandorina the conjugating 

 cells are of nearly equal size, in Eudorina an intermediate con- 

 dition is reached, while in Volvox the egg and sperm cells are 

 sharply differentiated in size and motility. Again, in the first 

 two and their allies all of the cells are at first vegetative and 

 afterward reproductive, while in Volvox the definite separation 

 into vegetative or somatic, and reproductive or germinal cells 

 makes its appearance. 



We arrive then at the conclusion, from the consideration 

 of these and many other lines of evidence, that the germ cells 

 were primitively exactly alike, and that the differences between 

 them have arisen in the process of differentiation along two 

 separate lines. Furthermore, it is clear that the differences 

 between the two sexes, which become strongly characterized 

 in the higher vertebrates, are all of a purely secondary nature. 



In their early development the germ cells are indistinguish- 

 able from each other, and both pass through certain stages, 

 preliminary to their union, which are essentially alike. The 

 animal egg is a large, more or less spherical cell, enveloped 

 usually by certain membranes, containing a large nucleus and 

 cytoplasm. The vast bulk of the egg cell, however, is made 

 up of inert food material in the form of yolk- granules, which 

 are stored up in it as nourishment for the developing embryo. 



