104 GENERAL EMBRYOLOGY 



production. But in all such cases the rate of formation of the germ 

 cells is rhythmic, increasing just before the breeding period. 



The sperm cells are always passed outside the body of the organism 

 forming them (save in self -fertilizing hermaphrodites); the eggs may 

 or may not be thus extruded. Animals used to be described (even clas- 

 sified) as "oviparous" or "viviparous," according to whether the female 

 extruded undeveloped eggs or living "young," but these terms have now 

 lost all precise meaning, for in any case eggs are formed, and in different 

 species the developing organisms may leave the body or reproductive 

 cavity of the parent at almost any stage. 



The unfertilized eggs may be simply thrown outside the body of the 

 female, as in most aquatic animals, the sperm being thrown out at the 

 same time and in approximately the same place ; in such cases fertiliza- 

 tion is ensured chiefly by the production of immense numbers of sperma- 

 tozoa. Such a process is very common among the Sponges, Coelen- 

 terates, Echinoderms, Annulata, Mollusca, Fishes, and many Amphibia. 

 Eggs thus thrown off into the water may float at or near the surface, as 

 pelagic eggs, or they may sink to the bottom among >the debris (demersal) . 

 Or the extruded eggs may be deposited with reference to definite and 

 often very special conditions affording, to the new organisms, protec- 

 tion, food, etc. Among land animals which deposit the eggs outside the 

 body, these are usually very definitely placed wfth reference to such 

 conditions ; the Insects afford a great variety of excellent illustrations of 

 relations of this kind. In some cases, among both aquatic and terres- 

 trial forms, definite "nests" are constructed in which the eggs are 

 deposited, and where the newly hatched organisms may remain for 

 some time. The eggs and young then may or may not be guarded 

 or fed, by either or both of the parents. The nests may vary from 

 simple depressions or pockets in the mud or sand, like those of many 

 fresh water Fishes, to the structures of very complex architecture of 

 many Birds. 



Among the forms which do not liberate their eggs at an early stage 

 in their development, there is a great variety of habit. In some Crus- 

 tacea and Amphibia, for example, the eggs are first extruded, but are 

 immediately placed upon the surface of the body, of either the male 

 or female parent, and develop there. Or they may become embedded 

 in the skin (many Amphibia) or may be deposited in some cavity not 

 primarily a reproductive cavity, such as the pharyngeal cavity, in some 

 of the Siluroid and Cichlid Fishes. In one of the Cyprinoid Fishes 

 (Rhodeus) the eggs are placed in the mantle cavity of a clam, where 

 they are fertilized and develop on the gills. In most cases where the 

 eggs are retained in "brood cavities," these are modified portions of 

 some part of the reproductive system proper ; here the eggs may remain 

 until a comparatively late period in their development. In such cases 



