REPRODUCTION AND DEVELOPMENT 



559 



Budding 



Budding is also an asexual process. Several 

 kinds of budding have been described in the 

 animals we have studied. Often, as in the 

 hydra, the bud grows out from the body, and 

 when completely differentiated, frees itself 

 from the parent; this is known as external 

 budding. The bud does not always resemble 

 the parent; for example, in the hydroid 

 Obdia, buds from the asexual polyp give rise 

 to sexual jellyfishes. In cases of polymor- 

 phism, as in the Portuguese man-o-war, 

 several types of individuals may arise by 

 budding from a single larva. A type of bud- 

 ding called internal budding, which results 

 in the formation of gemmules, is a normal 

 process in certain sponges and Bryozoa. 



REGENERATION 



Regeneration in invertebrates 



When an animal, such as the Parame- 

 cium, planarian, or an annelid, divides by 

 fission, each daughter reorganizes itself into 

 a normal complete animal. In nature, reor- 

 ganization may also occur in animals that 

 suffer injuries; parts may be lost, or the en- 

 tire animal may be cut in two. The wounds 

 of injured animals heal if they are not too 

 severe, large parts may be replaced, or a part 

 may reorganize itself into a complete normal 

 animal. This ability we call regeneration. 



Many interesting cases of regeneration 

 have been cited. Sponges are especially re- 

 markable, since an entire sponge may re- 

 generate from a group of dissociated cells. 

 Cuttings of sponges will grow and develop 

 into complete sponges much as cuttings of 

 plants do. The regenerative powers of the 

 hydra have been known for at least 200 

 years. Flatworms, such as the planaria, have 

 long furnished material for studies of re- 

 generation. How frequently earthworms in 

 nature lose parts of their bodies and regen- 

 erate them again is not known, but their 

 powers to do so occur largely through the 



activities of totipotent cells; these migrate 

 to the region of injury and differentiate 

 during regeneration. Legs, claws, eyes, an- 

 tennae, of the crayfish may be replaced if 

 lost. If the tentacle that bears the eye of a 

 snail is removed, a new tentacle with eye 

 may regenerate 20 times in succession in a 

 single animal. Starfishes are famous for their 

 ability to replace lost parts, and specimens 

 with regenerated arms are common in na- 

 ture. Sea cucumbers may lose a large part 

 of their internal organs, but they are able to 

 replace them in most cases. 



Regeneration in vertebrates 



Regeneration occurs in vertebrates as well 

 as in invertebrates. It is especially note- 

 worthy in salamanders where tails, limbs, 

 and eyes appear to be replaced easily. In one 

 experiment, the tail of a salamander was 

 removed and a new one regenerated 8 times 

 in succession. Lizards often escape enemies 

 such as birds, which grasp them by the tail; 

 the tail breaks off at a special breaking point; 

 the lizard escapes, and a new tail (with- 

 out bones) is regenerated. In man and many 

 other vertebrates regeneration is more re- 

 stricted. In many of the higher vertebrates, 

 the regeneration process is responsible for 

 the healing of external wounds, for the re- 

 newal of the epidermis and hair, and for the 

 production of new red blood corpuscles. 



It will be noted from this discussion that 

 the more primitive animals in both the 

 invertebrate and vertebrate series have the 

 greatest ability to regenerate lost parts. Also, 

 the power of regeneration is greater in 

 young than in old ones. 



Heteromorphosis 



The part regenerated is not always the 

 same as the part lost. This type of regenera- 

 tion is known as heteromorphosis. For ex- 

 ample, if an entire eye of a crustacean is cut 

 away, it may be replaced by an antennalike 

 structure. 



