204 DISEASES OF CATTLE. 



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 account 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 connec- 

 tion with the womb, and being furnished with abundant nourishment 

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

 and even two similarly formed bodies are attached by some point 

 common to both, as the navels, breastbones, back, etc. (Plate xix, 

 Figs. 1, 2, 3.) This shows 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 inde- 

 pendence 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 embryo, and is equally manifest in 

 the mature example of some of the lower forms of animal life. Thus 

 a newt will grow a new tail when that member has been cut oft', and a 

 starfish will develop as many new starfishes 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 comparable 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. 



The influence of disease in modifying growth in the early embryo, 

 increasing, decreasing, distorting, etc., is well illustrated in the experi- 



