GLOSSARY. 263 



Agamogenesis. Discontinuous development, as when the ova is 

 not brought into contact with the spermatozoa. 



Air-bladder. See Swim-bladder. 



Allaogenesis. A term used by Hackel to denote a form of pro- 

 duction in the Geryoniidae. (Now explained in another way.) 



Allantois. A foetal membrane, disappearing in the Mammalia at 

 an early period of foetal life, or else it is " placentiferous." In 

 function it is respiratory. 



Alternation of generations. First used by Steenstrup to designate 

 the phenomenon of an animal bringing forth a progeny not 

 resembling itself, but to whose descendants the resemblance 

 returns in the second, third, or fourth generation. One is an 

 act of reproduction, the other of development. A " successive 

 series of individuals" which "seem to represent two species 

 alternately reproduced" (Owen}. "An alternation of asexual 

 with sexual generation, in which the products of the one pro- 

 cess differ from those of the other" (Huxley}. " An interca- 

 lation of a proper sexual reproduction " is necessary in a true 

 alternation, according to Dr. Allman. See also Metagenesis. 



Alula. The small membranous appendages at the base of the 

 wings posteriorly in the Diptera. 



Alveoli. The sockets of the teeth in the Mammalia. In the Ea- 

 diolaria they are certain vacant spaces in the sarcode, placed 

 either within or without the capsule. 



Ambulacra. The perforated spaces for the emission of the tube- 

 feet (pedicelli) in the Echinodermata. The tube-feet them- 

 selves are sometimes called ambulacra, and that part of the 

 plate from which they issue ambulacral spaces. 



Ametabolous. Not undergoing any change. 



Amnion. A foetal membrane enveloping the embryo. It is 

 found only in reptiles, birds, and mammals. 



Amphiblastula. A stage in the development of sponges before 

 they become fixed. 



Amphicoelous. Said of vertebrae which are concave at both ends. 



Amphidiscs. " Two-toothed disks, like cogged wheels, united by 

 an axis" (Huxley}, forming a siliceous spiculum found in cer- 

 tain sponge-corpuscules. " Spicula which surround the gem- 

 mula of Spongilla" (Nicholson}. 



Amphigonous. When qualities or characters are transmitted from 

 both parents. 



Analogy. A similarity of functions without a similarity of parts. 



