GLOSSARY 



Abiogenesis. Also called spontaneous generation ; the doctrine that living 

 beings can arise de novo in the midst of not-living matter. Used in 

 contradistinction to biogenesis : the doctrine that every living cell, or 

 group of cells, is a link in the chain of life. 



Acinus. The blind grape- shaped termination of a gland or hollow organ. 

 The word alveolus is used with like signification. 



^Etiology. The doctrine of causes. Evolution and Natural Selection fall 

 under this head. 



Afferent Nerves. Those which convey waves of metabolic change inwards 

 to the central nerve-system ; in distinction from efferent nerves, 

 which convey these waves outwards to an organ or gland. 



Alecithal. A term applied to the ovum, such as the rabbit's, in which 

 there is little or no food-yolk, or in which the little there is, is dis- 

 tributed uniformly through the ovum. Used in contradistinction to 

 telolecithal (fowl's ovum), in which the food-yolk is aggregated at one 

 pole, and centrolecithal (crayfish ovum), in which it is aggregated at 

 the centre. 



Alternation of Generations. The alternation of two forms of the same 

 species. Under this general head come (1) metagenesis, when the one 

 form is sexual and the other asexual (medusa) ; (2) heterogamy, when 

 the second form (or forms) are perfectly or imperfectly sexual. The 

 liver-fluke falls under this head if we regard the masses of cells from 

 which the cercarise arise as rudimentary ovaries. When the ova 

 develop without the influence of spermatozoa, the term parthenogenesis 

 (maiden-birth) is used. 



Altrices. Birds whose young are hatched in a callow, immature condition, 

 as in the pigeon, as opposed to prcecoces, in which they are active at 

 hatching. 



Alveolus. See Acinus. 



Ametabola. Insects which undergo no metamorphosis, or in which the 

 metamorphosis is incomplete (sometimes called hemimetabola), as 

 opposed to holometabola, when the metamorphosis is complete. 



Amniota. Vertebrates in which the amnion, a f ratal membrane, is 

 developed (reptiles, birds, and mammals), in distinction from anam- 

 niota (frog, cod), in which there is no amnion. 



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