180 DEVELOPMENT AND HEREDITY. 



the stage where they consist of epiblast and meso- 

 blast, to divide by budding or constriction into a 

 number of separate embryos. Further, there is, in 

 this group, a truly wonderful variety of methods by 

 which the young animals grow out as buds from 

 the older ones. Now, a careful examination of all 

 these methods of budding shows that invariably 

 there enter into the composition of the bud, or new 

 individual, parts of both the original epiblast and 

 hypoblast, also of the mesoblast, if it be already 

 developed. Not only is this true of the budding 

 of the ascidians, but also of large groups of the 

 hydroids, and, I believe, wherever this method of 

 reproduction obtains. Apparently, each layer has 

 the power to develop, under favourable conditions, 

 into certain organs, and into these definite organs 

 only. Not as though one layer should produce an 

 arm and another a leg, but one layer produces the 

 muscles of the body, including muscles of both arm 

 and leg ; and another layer produces the skin of the 

 entire body, including arm and leg. Thus in each 

 germ-layer there is a singleness of plan which per- 

 mits each layer to produce only certain kinds of 

 tissue. 



We see, therefore, that, at a very early stage in 

 the development of the embryo, the parts which are 

 to develop into the various systems of organs become 



