AX INTRODUCTION TO THE STUDY OF GROWTH. 3! 



possible, through change in the conditions surrounding 

 it, to cause the development within it of more than one 

 embryo. (Fig. 6.) 



In these experiments the attempt is made to grow an 

 animal from a cell or a part of a cell, which represents 

 only a fraction of the substance naturally intended for 

 the entire organism, and as one consequence the embryo 

 is below the normal size. To interpret such phenomena 

 in harmony with the idea of preformation in the egg, 

 is a present line of zoological endeavour ; but, whatever 

 may be the outcome, the striking fact remains, that such 

 repair as occurs when complete embryos are reared from. 

 less than the entire egg, is not quantitative, because the 

 size of the embryo bears a definite relation to the size of 

 the original fragment. 



In this connection it may be asked why animals grow 

 to that particular size which is characteristic for the 

 different species ; why one egg yields a mouse, another 

 an elephant ? But until more facts on the size of egg- 

 cells are available for comparison, speculation on the 

 subject would be futile. 



Young and growing cells have a nucleus which is 

 large when compared with the amount of cytoplasm, 

 and the possibilities of growth and division in animal 

 cells appear to be dependent on this relation. If this 

 condition is a necessary one, it should be found univer- 

 sally, even where growth, due to the regeneration of the 

 lost parts, is taking place. For not only is the ovum 

 capable of producing the one complete animal, but, 

 in addition, retains, in many forms, the power to 

 reproduce lost parts. This process is the more interest- 

 ing in view of the fact that among the invertebrates 

 generally, and in many lower vertebrates also, especially 

 when young, the regeneration of the lost parts seems so 

 simple and common an occurrence, and can take place 



