GERM CELLS AND THEIR FORMATION 103 



breed or spawn at shorter intervals. In many Mammals it is true that 

 there is only a single annual breeding season or period of cestrus; this 

 condition, known as moncestrous, is characteristic of most Carnivora 

 and is found also in the Chiroptera and Marsupials. Others, however, 

 are polycestrous, and exhibit two or three annual breeding seasons 

 (Insectivors), and in still others the period of oestrus may occur at inter- 

 vals of a few weeks (man), or it may be quite continuous, as in most 

 Rodents and some Carnivors. (See Marshall, Physiology of Reproduc- 

 tion, 1910.) 



Among the higher animals the breeding season is often preceded by a 

 "nuptial season" during which, especially among the males, there may 

 develop various special morphological and physiological peculiarities. 

 The Fishes, Birds, and Mammals exhibit the frequent development of 

 special external markings or colorings, special secretions, and unusual 

 modes of behavior. Both these and the breeding habits proper, are to 

 be regarded as responses to stimuli, frequently climatic in origin, 

 resulting from changes in temperature, light, moisture, food characters, 

 etc. These phenomena are commonly regarded as indications of an 

 increased metabolism that affects not only the organs of reproduction, 

 but secondarily the whole body. 



In a few rare instances, chiefly among the segmented worms, the an- 

 nual spawning season is very definitely fixed and varies within limits 

 of only a few calendar days. More usually the time of spawning is 

 subject to wide variation and is dependent upon temperature and other 

 seasonal conditions. The species of the Palolo worm afford one of the 

 best marked instances of a fixed spawning season. It is not quite as 

 regular and limited as tradition would have it, but in the Tortugas, 

 most of the individuals of the Atlantic Palolo (Eunice fucata) swarm and 

 spawn during one or two mornings which fall within three days of the 

 moon's last quarter between the latter part of June and the end of July 

 (Mayer). The Pacific species (E. viridis) spawns similarly on and near 

 the last quarter in October and November. Somewhat similar relations 

 have been determined for other Annulata, such as Amphitrite (Scott) 

 and Ceratocephale. The determining factor in these and similar cases 

 seems to be the character of the tides, combined with factors of tempera- 

 ture and light. Other organisms spawn at a definite time of day, 

 individuals coming to maturity at any time during a longer breeding 

 season. Thus Amphioxus and some Hydroids spawn only about sun 

 down or shortly thereafter. 



During the intervals between the breeding periods the formation of 

 the germ cells may almost or quite cease, to recommence shortly prior 

 to the next period. In some creatures, however, the eggs are formed 

 continuously and are stored in secondary reproductive cavities pending 

 the time of their production. This is more likely to be the case in sperm 



