208 LIFE: ITS NATURE AND ORIGIN 



They may, for example, lack certain organs or parts or have too 

 many limbs. I have seen a bull with two heads, one quite small 

 and imperfect. A well-known instance is the heritable abnormal- 

 ity otocephaly (loss of head) in guinea-pigs found by Sewall 

 Wright, 56 which probably results from inbreeding. While the rest 

 of the body is normal, the following graded degrees of interference 

 with head formation are found, which F. E. Lehmann 57 believes 

 to be due to a defect in the head organizer: in Figure 31 (1) lower 

 jaw is reduced; (2) no mandible palpable externally; (3) ears con- 

 nected under throat by bare skin; (4) single median ear opening 

 on throat; (5) mouth and upper incisors lost; (6) nostrils fuse; 

 (7) eyes are in contact below a narrow nasal probosis or are more 

 or less fused; (8) eye fusion complete; (9) probosis lost; (10) eye 

 lost; (11) ear opening lost; (12) body rounds off in front of 

 shoulders, with no sign of a head except a small single median 

 external ear. 58 



Goldschmidt states that the various defects which combine to 

 make otocephaly are traceable to a small number of centers of ab- 

 normality: ventral mandibular arch, olfactory placodes, cerebral 

 vesicles, median optic rudiment and some others. 



Embryological investigation of the heritable x-ray induced 

 mutations produced in mice by C. C. Little and H. J. Bagg 59 made 

 by K. Bonnevie 60 and G. M. Plagens 61 showed that while the 

 abnormalities may be much diversified, e.g., lack of eye (anoph- 

 thalmia), extra digits (polydactyly), club foot, etc., they follow 

 as a consequence of an overflow of medullary fluid in the neck of 

 the embryos when they are 7 or 8 milimeters in length. The blebs 

 or blisters thus produced move about uncertainly as embryonal 

 growth continues, and interfere with normal development. What 

 the recessive mutant gene does is simply to establish the conditions 

 leading to the release of the medullary fluid; and since this hap- 

 pens at an early stage, quite a variety of final effects may follow. 



Similarly, it is uncertain what may be the consequences of an 

 embolism in the circulatory system of a human being. It might 

 kill by obstructing the circulation in the heart, the brain, the 

 lungs, etc. Hence the wisdom of absolute rest in phlebitis, so 

 that no dangerous blood-clot is set free while redispersion of the 

 large local clot is proceeding. 



We now mention a few of the many cases where developmental 

 abnormalities have been experimentally produced by the presence 

 or the lack of definite chemical substances. 



