564 



COLLEGE ZOOLOGY 



is produced in one animal; in some other 

 animals, the number appears to be indefi- 

 nite. In Volvox (Fig. 22) and in sponges, 

 the gametes are scattered, and no definite 

 gonads are recognizable; but in the hydra 

 (Fig. 50), the gonads consist of groups of 

 interstitial cells which form conical (male) 

 or ball-shaped (female) projections on the 

 stalk. In the hydra, eggs and sperms need 

 no ducts in order to escape from the body, 

 Flatworms, earthworms, and other higher in- 

 vertebrates and vertebrates possess gonads, 

 in which the eggs and sperms develop, and 

 ducts for carrying them to the outside. These 

 essential reproductive organs are often very 

 complicated; those of the various types may 

 be found by looking in the index under the 

 names of the different animals, or under the 

 term "reproduction." 



Reproductive ducts and glands 



The reproductive ducts of the female are 

 usually one or two oviducts that carry the 

 eggs from the gonads (ovaries) directly to 

 the outside, or to a cloaca, uterus, etc. The 

 male ducts are usually one or two vasa 

 deferentia which carry the sperms directly 

 to the outside, or into an ejaculatory duct, 

 etc. The male ducts are often enlarged near 

 the posterior end into seminal vesicles (Fig. 

 224), where sperms are stored until needed; 

 and seminal receptacles may be present in 

 the female for a like purpose. In many ani- 

 mals, glands are associated with the gonads 

 and their ducts, as for example, yolk glands 

 in planarians (Fig. 68); cement glands in 

 insects; oviduct glands that secrete the gela- 

 tinous coat of the frog's egg (Fig. 223); 

 glands that secrete the albumin, shell mem- 

 brane, and shell of the bird's egg; and the 

 prostate and Cowper's glands that add their 

 secretions to the spermatozoa in mammals. 



Fertilization and egg laying 



Both structures and activities are fre- 

 quently combined in order to insure fer- 

 tilization of the eggs of different animals. 



In many aquatic animals, such as fish and 

 frogs, the eggs are laid directly in the water, 

 and sperms either come in contact with 

 them by chance or are poured over them by 

 the male, as in the frog. In other aquatic 

 animals, for example the crayfish and in 

 most terrestrial animals, the sperms are in- 

 jected into the female by the male, where 

 they unite with the eggs at once, or are 

 stored until required. Animals that lay their 

 eggs either before or after fertilization are 

 oviparous. In certain types of animals the 

 eggs are retained in the body until they 

 hatch, as in certain sharks, fish, amphibians, 

 lizards, and snakes; the term ovoviviparous 

 is applied to this procedure. The fertilized 

 egg in most mammals is nourished within 

 the body by food from the blood of the 

 mother that passes through a special em- 

 br}'onic organ called the placenta. Mammals 

 of this type and other animals that give 

 birth to young that develop from eggs within 

 the body of the mother and are nourished 

 from her blood stream are called viviparous. 



EMBRYOLOGY 

 (DEVELOPMENT) 



Descriptive embryology 



Embryology is the subdivision of biology 

 which deals with the fertilized egg and its 

 development. Historically, embryologists 

 were interested initially in describing the 

 visible changes in the fertilized egg as it 

 develops, and in comparing the develop- 

 mental events observed in the egg of one 

 species with those observed in the eggs of 

 other species. The first visible event in de- 

 velopment is the subdivision of the fertil- 

 ized egg into a number of cells, the process 

 of cleavage (Figs. 41, 56, 237, 258, 350, and 

 349). Sooner or later the individual cells 

 ( blastomeres ) rearrange themselves individ- 

 ually around a cavity called the blastocoel 

 (Figs. 41, 56, 237, and 349); this constitutes 

 the blastula stage. Subsequently groups of 

 blastomeres shift their positions in such a 



