SEXUAL REPRODUCTION. 935 



Entozoa, but not amongst animals higher in the scale. In artificial 

 fission, as performed upon the Hydra, a similar process is imitated. 

 If, for example, a Hydra be cut, lengthwise or transversely, into 

 several parts, each portion will complete itself. Some of the Annel- 

 ida, or Worms, on being cut across, develop a new head to the lower 

 half, and a new tail to the upper portion, of the divided body. 



Gemmation, or gemmiparous reproduction, consists in the formation 

 of an offshoot or bud, from the body of the parent animal, which 

 either continues to grow in connection with the parent, so that com- 

 posite animals are produced, as the many-chambered Rhizopods : or 

 aggregates or colonies of animals are formed, attached by a common 

 stem, or stolon, as in the case of the Vorticellse, amongst the Infusoria. 



This is, also, the ordinary mode of propagation of the Hydra, 

 amongst the Coelenterata ; of the compound coralline Polyps, of the 

 compound Ascidioida, and of Nais, amongst the Annelida. Gremmse 

 may also, after a time, detach themselves from the parent stem, move 

 away, and develop as independent animals, which may themselves 

 gemmate, and form a new colony, as in the compound Polyps and the 

 compound Tunicata, or become new independent animals, as in the 

 Medusae. Sponges are thus reproduced by detached bodies, known as 

 gemmules, which, at first ciliated and free-moving, afterwards become 

 smooth and fixed, and then grow into a new sponge. 



As representing a special form of internal gemmiparous reproduc- 

 tion, may perhaps be included those remarkable cases, in which groups 

 of minute cell-like bodies, sometimes named pseudova, to distinguish 

 them from true ova or eggs, are developed somewhere in the interior 

 of the body of the parent animal, and, after a time, undergo successive 

 stages of development, sometimes into forms externally resembling 

 that of the parent animal, though not possessing reproductive organs, 

 but much more commonly into forms not resembling the parent animal. 

 By detachment, protrusion, or rupture of the parent, these new ani- 

 mals then become independent beings. This form of reproduction is 

 observed in the Aphides only, among Insects, and in the Daphnia, 

 among the smaller Crustacea. Light and heat are important agents 

 in determining the occurrence of this process. 



The larger reproductive bodies found in the Sponges, and developed 

 as cold weather approaches, called capsules, have, by some, been re- 



farded as of the nature of pseudova, but they may be sexual products, 

 he so-called germ-cells of the Hydra, which, towards winter, are 

 sometimes developed in the walls of the gastric cavity in one individual, 

 whilst a sperm-cell appears in that of another, though sometimes both 

 kinds of cells are developed in the same Polyp, have likewise been 

 referred to this class of bodies ; but their sexual character is more 

 probable. 



In the sexual mode of reproduction, known as oviparous reproduc- 

 tion, which is a higher form of propagation, an ovum, or germ-cell, and 

 a fertilizing cell, or sperm-cell, are always necessary, and co-operative, 

 in other words, a female and a male product, according to the distinc- 

 tion of sex. 



In some animals, as in the Coelenterata, in certain Scolecida, as in 



