102 GENERAL EMBRYOLOGY 



passive egg material. This differentiation affords the material 

 basis for development while at the same time it ensures the 

 fertilization of practically every egg produced, though the eggs 

 and sperm may be shed freely into the water some distance apart, 

 a distance often very great as compared with the size of the 

 cells. The relatively very large amount of cytoplasm in the 

 egg, and small amount in the sperm, constitute the most 

 marked exceptions to the nucleo-cytoplasmic (kern-plasma) 

 relation, mentioned in the preceding chapter; these conditions 

 are entirely special and are to be regarded as adaptations to 

 the very unusual functions of these cells. 



The details concerning the form and number of the sperm and egg 

 cells, the amount of yolk in the eggs, the character of their membranes, 

 etc., are significant only from the viewpoint of adaptedness to the con- 

 ditions under which they must function. This adaptedness of the re- 

 productive phenomena toward ensuring the final bringing to maturity 

 of a number of organisms sufficient to maintain the specific group 

 in undiminished numbers is a general biological topic of especial interest. 

 In strictness this lies outside our province, but to omit entirely 

 any reference to this subject, leaves without significance many of 

 the details of structure and behavior mentioned in the preceding 

 paragraphs. We may therefore suggest briefly a few of these relations, 

 not only as regards the germ cells, but also the general processes of 

 spawning, etc., which are all concerned in finally bringing together an 

 ovum and a spermatozoon. 



All these varied, and often complex, phenomena of habit and morpho- 

 logical specialization of the reproductive cells are correlated with the 

 special conditions of life which affect the chances that a single egg shall 

 finally become a mature organism. They are conveniently grouped 

 under three chief heads: (a) the ensurance of mating, (b) the ensurance 

 of the actual meeting and fusion of the germ cells, (c) the chances of death 

 before maturity, involving such factors as abundance of food, enemies, 

 adverse conditions in the inorganic surroundings, necessity for reaching 

 special conditions of development, food, etc., duration of the period of 

 development, and the like. 



A few forms, especially in the warmer climates, appear to breed quite 

 continuously throughout the year (many Coelenterates, Mollusca, etc.), 

 but commonly the germ cells are produced at regular periods, which may 

 have a duration of only a few days or hours, or they may extend over 

 several months. Breeding or spawning periods are nearly always sea- 

 sonal and usually annual, but a few forms, particularly the Mammals, 



