4 o POPULAR SCIENCE MONTHLY. 



attached to each other and form a cluster or a chain, or they may sep- 

 arate and become independent isolated cells. The multiplication, 

 under favorable conditions of light, air, temperature, moisture and food, 

 goes on with extraordinary rapidity, so that in a few hours many 

 thousand new individuals may arise from a parent bacterium. 



Connected with the life-history of a bacterium cell is the formation 

 in its substance, in many species and under certain conditions, of a 

 highly refractile shiny particle called a spore. At first sight a spore 

 seems as if it were the nucleus of the bacterium cell, but it is not always 

 present when multiplication by cleavage is taking place, and when 

 present it does not appear to take part in the fission. On the other 

 hand, a spore, from the character of its envelope, possesses great power 

 of resistance, so that dried bacteria, when placed in conditions favorable 

 to germination, can through their spores germinate and resume an ac- 

 tive existence. Spore formation seems, therefore, to be a provision for 

 continuing the life of the bacterium under conditions which, if spores 

 had not formed, would have been the cause of its death. 



The time has gone by to search for the origin of living organisms by 

 a spontaneous aggregation of molecules in vegetable or other infusions, 

 or from a layer of formless primordial slime diffused over the bed of the 

 ocean. Living matter during our epoch has been, and continues to be, 

 derived from pre-existing living matter, even when it possesses the sim- 

 plicity of structure of a bacterium, and the morphological unit is the 

 cell. ' 



DEVELOPMENT OF THE EGG. 



As the future of the entire organism lies in the fertilized egg cell, we 

 may now briefly review the arrangements, consequent on the process of 

 segmentation, which lead to the formation, let us say in the egg of a 

 bird, of the embryo or young chick. 



In the latter part of the last century, C. F. Wolff observed that the 

 beginning of the embryo was associated with the formation of layers, 

 and in 1817 Pander demonstrated that in the hen's egg at first one layer, 

 called mucous, appeared; then a second or serous layer, to be followed by 

 a third, intermediate or vascular layer. In 1828 von Baer amplified our 

 knowledge in his famous treatise, which from its grasp of the subject 

 created a new epoch in the science of embryology. It was not, however, 

 until the discovery by Schwann of cells as constant factors in the struc- 

 ture of animals and in their relation to development that the true nature 

 of these layers was determined. We now know that each layer consists 

 of cells, and that all the tissues and organs of the body are derived from 

 them. Numerous observers have devoted themselves for many years to 

 the study of each layer, with the view of determining the part which it 

 takes in the formation of the constituent parts of the body, more es- 



