A YORKSHIRE NATURALIST 121 



use. That formed in the cartilage I have called "chon- 

 driform " bone, and that in the more external layers 

 of membrane, " membraniform " bone. In the mam- 

 malia, nearly, but not quite, all the former tissue 

 disappears at a very early age, the growth of the 

 skeleton being effected by the addition and subse- 

 quent ossification of layer after layer of periosteum. 



Minute cavities, called lacunae, abound in each 

 layer of membraniform bone, in all mammalia and 

 reptiles. The question of the origin of these lacunae 

 brings before us a remarkable epoch in the history 

 of biology, viz., the fourth decade of the present 

 century. Previously to that date the subject of the 

 unity of the organic world was altogether misunder- 

 stood. Thus, prior to the period referred to, it was 

 supposed that the development and growth of animal 

 bodies were brought about by processes wholly 

 different from those which produced the same results 

 in the vegetable world, an error which led to endless 

 vague speculations and baseless guesses. Much was 

 gained when thinkers arrived at the sound conclu- 

 sions of omne vivum ex viva and omne vivum ex 

 ovo. Both these axioms struck at the root of the 

 nonsense prevalent under the name "Spontaneous 

 Generation." 



But a still greater end was attained when we 

 learnt the signification of the abiding axiom of omne 

 vivum ex celluld. Then it was that we understood 

 for the first time the true seat of life and its uninter- 

 rupted transmission from generation to generation. 



