i894. ''PREFORMATION OR NEW FORMATION:' 295 



division, differences arise in the cells according to their position and 

 relations to each other, and the cells are different quahtatively from 

 the original cell from which they sprang. 



In many respects the form assumed by the cell-masses is a 

 function of organic growth. The original cell, from the physical 

 nature of the protoplasm, naturally has a spherical shape, and the 

 retaining of this shape by the cell-mass produces differences in the 

 shapes and surfaces of the constituent cells. The next important 

 factor is the necessity that growth shall take place so as to secure the 

 greatest possible extension of surface in proportion to mass, A 

 crystal grows by the deposition of new particles upon the outside ; 

 organic matter grows by the intersusception of matter from the 

 different constituents in the environment, and it is therefore necessary 

 that as large a surface as possible shall be exposed to the environ- 

 ment in order that the varied substances may be picked up. 



The organic material in process of growth can assume only those 

 forms which allow it to remain in touch with the outer world. A cell- 

 mass could not grow indefinitely thick without the inner cells being 

 deprived by the outer cells of their relations to the environment. For 

 this reason, cell-multiplication produces threads and layers and mem- 

 branes, rather than solid masses. In the plant kingdom the cells, for 

 the most part, are strengthened by a firm cell-wall, through which 

 their liquid and gaseous food is able to pass. The possession of this 

 cell-wall enables plant growth to remain, for the most part, in the 

 form of threads and membranes, and the necessary extension of 

 surface is produced in the simplest possible way. In the adult plant 

 all the organs and tissues grow out so as to preserve this simple 

 extension of surface. In the animal kingdom the method of nutrition 

 by the absorption of solid masses into the interior of the cells prevents 

 the formation of thick cell-walls. Animal growth, then, cannot rely 

 upon the nature of the individual cells to secure the necessary rigidity 

 and coherence of the whole mass. The cell-masses grow so as to form 

 hollow bladders and tubes, the whole structures possessing a rigidity 

 not found in the individual cells, and growth takes place by the 

 infolding of the single layers of cells which form the walls of the 

 spheres and tubes. In plant-tissues the food-supply comes from the 

 water and the air of the outer world, and in consequence the exten- 

 sion of surface is as much as possible an extension turned to face the 

 outer world. In animals the solid food is taken into an internal 

 cavity, where it is broken up and ingested, and in consequence 

 there is as great as possible an extension of this inner surface. 

 Thus, the blastula is a hollow sphere, the walls of which consist 

 of a single layer of cells which retain contact with the environ- 

 ment on their inner and outer surfaces. An inpushing of the 

 wall of the blastula gives rise to the gastrula, and subsequent 

 infoldings in the inner and outer walls of that give rise to muscle 

 layers, nerve layers, ccelomic pouches, and so forth. But in ail these 



