THE BREEDING BEHAVIOR OF ANIMALS 



181 



germ cells are produced at different times, eggs first in some, spermatozoa 

 first in others. Obviously there can be no self-fertilization under these 

 circumstances. In still others, the mating habits prevent self-fertiliza- 

 tion, as has been described for the earthworm in the preceding chapter. 

 Some of the roundworms, however, and Sacculina, which is a parasite 

 attached to crabs, fertilize their own eggs regularly. No special act is 

 necessary to bring eggs and spermatozoa together in these self-fertilizing 

 forms, since they mingle freely within the body. Sometimes self-fertiliza- 

 tion may occur accidentally, as in Hydra whose sperms are shed into the 

 water, where they penetrate eggs still located in the ovaries. Since the 

 spermatozoa find the eggs largely by chance, they may reach either an 

 egg in the same individual or one of another individual. 



Fertilization in Dioecious Species. — In many aquatic animals with 

 separate sexes the sexual elements, or at least the spermatozoa, are 

 simply discharged into the water and the germ cells come together by 

 chance. Thus in the jellyfishes the spermatozoa are liberated into the 

 water and may or may not happen to meet the eggs, which are retained 

 in the ovaries of the females of some species just as in Hydra. In other 

 animals there is congregation of the sexes at the breeding time, and the 

 eggs and spermatozoa are liberated in 

 proximity. Starfishes and sea urchins 

 periodically congregate in this manner. 

 This close association of the sexes un- 

 doubtedly greatly favors the meeting of 

 the germ cells but still leaves to chance 

 an important role, and many eggs are 

 never fertilized. The hellbender (a sala- 

 mander) is a form that congregates with 

 its fellows at the breeding season. In 

 certain other salamanders the male 

 deposits the spermatozoa in a naked, 

 nearly spherical mass resting on a 

 gelatinous stalk which is attached to a 

 leaf or some other object in the water. 

 This structure, including the stalk, is 

 called a spermatopho)-e (Fig. 150). The mass of spermatozoa at its top is 

 subsequently removed by the female with the lips of the cloaca, and the 

 eggs are fertilized within her body. 



This last way of bringing eggs and spermatozoa together can be 

 adopted only by animals that fertilize their eggs internally. In frogs 

 and toads, fertilization occurs outside the body, and in these forms special 

 behavior is designed to bring the sex cells together. In addition to 

 congregating at the breeding season, the males practice clasping. The 



Fig. 150. — Spermatophore of No- 

 tophthalmus viridescens viridescens 

 (Raf.), the common newt of eastern 

 North America. The stalk is a clear 

 gelatinous substance; the apical mass 

 (dotted in the figure) is a snowy- 

 white mass of seminal fluid contain- 

 ing spermatozoa. {After B. G. 

 Smith.) 



