THE CELLULAR CHANGES OF AGE 47 



A correct understanding of the conception cytomor- 

 phosis is an indispensable preliminary to any com- 

 prehension of the phenomena of development of 

 animal or plant structure. I shall endeavour, there- 

 fore, now to give you some insight into the phenomena 

 of cytomorphosis as regarded by the scientific bio- 

 logist. The first cells which are produced are those 

 which form the young embryo. We speak of them 

 on that account as embryonic cells, or cells of the 

 embryonic type. Our next picture illustrates the 

 actual character of such cells as seen with the micro- 

 scope, for it represents a series of sections through 

 the body of a rabbit embryo, the development of 

 which has lasted only seven and one half days. You 

 will notice at once the simplicity of the structure. 

 There are not yet present any of those parts which 

 we can properly designate as organs. The cells have 

 been produced by their own multiplication, and are 

 not yet so numerous but that they could be readily 

 actually counted. They are spread out in somewhat 

 definite layers or sheets, 1 but beyond that they show 

 no definite arrangement which is likely to attract 

 your attention. That which I wish you particularly 

 to observe is that in every part of each of these sec- 



1 The layers are three in number, and are known as the germ-layers; the 

 outer layer (uppermost in each of the three sections in Fig. 9) is the ectoderm, 

 the middle the mesoderm, and the inner one the entoderm. The science of 

 embryology has for its chief task to trace the numerous modifications and often 

 very complicated metamorphoses which the three simple germ-layers pass 

 through in order to produce the complex organs of the adult. Our present 

 conceptions of the structure of multicellular animals are based on two great 

 discoveries, first of the germ-layers, second of cells. 



