DISEASES OF THE GENERATIVE OEGANS. 185 



Other monstrosities seem to have started in too close breeding, by 

 which the powers of symmetrical development are impaired, just as 

 the procreative power weakens under continuous breeding from the 

 closest blood relations. A monstrosity consisting in the absence of 

 an organ often depends on a simple lack of development, the result 

 of disease or injury, as a young bone is permanently shortened by 

 being broken across the soft part between the shaft and the end, the 

 only part where increase in length can take place. As the result of 

 the injury the soft growing layer becomes prematurely hard and 

 all increase in length at that end of the bone ceases. This will ac- 

 count for some cases of absence of eye, limb, or other organ. 



Sometimes a monstrosity is due to the inclosure of one ovum in 

 another while the latter is still but a soft mass of cells and can easily 

 close around the first. Here each ovum has an independent life, and 

 they develop simultaneously, only the outer one having direct con- 

 nection with the womb and being furnished with abundant nourish- 

 ment advances most rapidly and perfectly, while the inclosed and 

 starved ovum is dwarfed and imperfect often to the last degree. 



In many cases of excess of parts the extra part or member is mani- 

 festly derived from the same ovum, and even the same part of the 

 ovum, being merely the effect of a redundancy and vagary of growth. 

 Such cases include most instances of extra digits or other organs, and 

 even of double monsters, as manifested by the fact that such extra 

 organs grow from the normal identical organs. Hence the extra 

 digit is attached to the normal digit, the extra head to the one neck, 

 the extra tail to the croup, extra teeth to the existing teeth, and 

 even two similarly formed bodies are attached by some point com- 

 mon to both, as the navels, breastbones, back, etc. (PI. XIX, figs. 

 1, 2, 3.) This sliows that both have been derived from the same 

 primitive layer of the embryo, which possessed the plastic power of 

 building up a given structure or set of organs. An inclosed ovum, 

 on the other hand, has no such identity or similarity of structure to 

 the part with which it is connected, showing an evident primary 

 independence of both life and the power of building tissues and 

 organs. The power of determining extra growth along a given 

 natural line is very highly developed in the early embn^o and is 

 equally manifest in the mature examples of some of the lower forms 

 of animal life. Thus a newt will grow a new tail when that member 

 has been cut off, and a starfish will develop as many new stai-fishes 

 as the pieces made by cutting up the original one. This power of 

 growth in the embryo and in the lower forms of animals is compa- 

 rable to the branching out again of a tree at the places from which 

 branches have been lopped. The presence of this vegetable-like 

 power of growth in the embryo accounts for most double monsters. 



