THE GAMETES IN PLANTS AND ANIMALS 349 



THE PHYSIOLOGICAL CONDITION OF THE GAMETES 



If the gametes are highly differentiated cells the final stages of 

 i. period of growth and progressive development they must be 

 physiologically in an advanced stage of senescence. Their rate of 

 metabolism and rate of growth must have been high at the beginning 

 of the period of differentiation and have undergone decrease during 

 this period. 



As regards these points, our positive experimental knowledge 

 is very slight, but various facts of observation point very clearly 

 to certain conclusions. Growth has ceased in the fully developed 

 gametes, but the earlier stages of their development are periods of 

 rapid and extensive growth, and in the plants there is usually more 

 or less cell division in the earlier stages of gametic development. 

 In the female gamete growth is usually considerable, often very 

 great in amount. In many of the lower plants the cytoplasm of 

 the egg becomes loaded with nutritive substance, as in the case of 

 the yolk- bearing animal egg, but in the higher plants development 

 follows a different course and the embryo obtains its food to a large 

 extent from other cells. The rate of growth in the developing 

 gamete is apparently higher in the earlier than in the later stages, 

 but I am unable to cite any exact observations upon this point. 



The course of development in the male gamete usually differs 

 very widely from that in the female. Growth occurs, but it is much 

 less in amount, and instead of the accumulation of inactive sub- 

 stance in the cytoplasm, a transformation of the cytoplasm into a 

 morphological mechanism, usually motor in function, occurs. In 

 the fully developed male gamete, as in the female, growth has 

 ceased. In most cases the general metabolic substratum of the 

 cell has to a large extent or wholly disappeared and the cell has very 

 evidently progressed as far as is possible in a certain direction. 



As regards the metabolic condition of the gametes of plants, 

 G. Maige ('09, 'u) has shown that the rate of respiration decreases 

 in the anther during the development of the pollen grain from the 

 spore. The rate of respiration in the pistil, however, is usually 

 higher than in the anther and frequently increases during the 

 development of this organ. The changes of rate in the embryo sac 

 alone have not been determined, but it seems probable that the 



