390 



SCIENCE. 



[K S. Vol. XII. No. 298. 



terium as formed of protoplasm containing 

 granules capable of being colored, which 

 are a part of the protoplasm, itself, and not 

 a nuclear substance. On the latter view, 

 bacteria would consist of cell plasm enclosed 

 in a membrane and destitute of a nucleus. 

 Whatever be the nature of the granule- 

 containing material, each bacterium is re- 

 garded as a cell, the minutest and simplest 

 living particle capable of an independent 

 existence that has yet been discovered. 



Bacteria cells, like cells generally, can 

 produce their kind. They multiply by 

 simple fission, probably with an ingrowth 

 of the cell wall, but without the karyo- 

 kinetic phenomena observed in nucleated 

 cells. Each cell gives rise to two daughter 

 cells, which may for a time remain attached 

 to each other and form a cluster or a chain, 

 or they may separate and become independ- 

 ent 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 bac- 

 terium cell is the formation in its substance, 

 in many species and under certain condi- 

 tions, 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 mul- 

 tiplication 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 active ex- 

 istence. 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 simplicity 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, interme- 

 diate or vascular layer. In 1828 von Baer 

 amplified our knowledge in his famous 

 treatise, which from its grasp of the sub- 

 ject created a new epoch in the science of 

 embryology. It was not, however, until 

 the discovery by Schwann of cells as con- 

 stant factors in the structure of animals 

 and in their relation to development that 

 the true nature of these layers was deter- 

 mined. We now know that each layer 

 consists of cells, and that all the tissues 

 and organs of the body are derived from 

 them, l^umerous 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- 

 pecially in the higher animals, and the im- 

 portant conclusion has been arrived at that 

 each kind of tissue invariably arises from 

 one of these layers and from no other. 



