SUBK1NGDOM METAZOA. 59 



sionallj linear colonies are formed which are with difficulty 

 to be distinguished from those formed by division. Indeed 

 a definite distinction between budding and division is not 

 possible, though where an alternation of older and younger 

 individuals occurs in a linear colony division is indicated, 

 while in one produced by budding there is a regular succes- 

 sion of gradually older individuals from before backwards. 



Closely related to budding is the power of regeneration 

 of parts. The higher Crustacea possess an extraordinary 

 power of regenerating lost limbs, and provision is present in. 

 crabs and the lobster for the self-amputation of a limb when 

 such a mutilation seems to be demanded by the exigencies 

 of the situation. In the lower forms, however, the extent to 

 which such regeneration may be carried is much greater, ex- 

 tending even to the reproduction of the whole by a compara- 

 tively small part. A Starfish is not only able to regenerate 

 an arm which has been accidentally lost, but from an arm and 

 a portion of the disk all the missing parts may be developed ; 

 and Hydra or a Sponge may be divided into a large number 

 of pieces each of which is capable of developing into an entire 

 animal. Such phenomena, as well as budding and division, 

 depend either upon a low degree of differentiation of the 

 tissues, as in such a form as a Sponge or in Hydra, or else to 

 the persistence of a certain amount of tissue in an embryonic 

 or undifferentiated condition. In a Bryozoau bud, for in- 

 stance, as its tissues gradually differentiate into the adult 

 condition, a number of cells lag behind and do not take part 

 in the differentiation, and later give rise to a new bud ; and 

 similarly in the Annelid worms the tissues of a regenerating 

 part show an appearance and mode of differentiation similar 

 to what they present in the development from the ovum. 

 Conversely, the greater the degree of differentiation and in- 

 tegration of the tissues and organs of an animal the less 

 is the power of regenerating lost parts or of reproducing by 

 budding. 



As a general rule ova are incapable of developing into the 

 adult form unless fertilized by a spermatozoon. In a number 

 of forms, however, a development of unfertilized ova occurs 

 constituting a mode of reproduction known as partheno- 



