450 PEOP. B. C. A. WINDLE ON XERATOLOGICAL EVIDENCE 



viz. excess or defect. To the former class belong double mon- 

 strosities, complete and incomplete, unilateral or partial hyper- 

 tropbies, and excesses sucb as Polydactyly (at least tbe majority 

 of cases), polymastia, with others, often not recognized unless a 

 dissection happens to be made, such as accessory carpal bones, 

 splenculi, &c. Having discussed the sources of this excess of 

 material in a former paper*, I shall not delay here further on 

 that point. To use Weismann's term, malformations of this class 

 are, with probably very few exceptions, inherent in the germ or 

 blastogenic. Deficiency of material leads to defect of size or 

 parts. Thus, to this group belong dwarfs and some cases of 

 ectromelia, ectrodactyly, microphthalmus, and anophthalmus, with 

 possibly most of the non-mechauically caused clefts, such as 

 cleft-palate, persistent branchial cleft, and coloboma oculi. How 

 is this deficiency of material caused ? I believe in at least two 

 ways : — (1) By faulty segmentation during the extrusion of the 

 polar bodies, whereby too much formative material is got rid of. 

 (2) By the very early destruction by some morbid process of 

 early segmentation-spheres. Eoux's experiments, which I have 

 quoted in my paper already mentioned, and an account of wliich 

 will also be found in a recent address by Mr. Poulton f, show 

 that even the first few lines of segmentation cut off" morphological 

 areas the destruction of any one of which would lead to the non- 

 development of the part which it was destined to form. It is, 

 of course, possible that to the two preceding causes should be 

 added that of an original paucity of material in ovum or sper- 

 matozoon prior to the extrusion of the polar bodies. The first 

 and last causes would be inherent in the germ, and therefore 

 blastogenic ; but what of the second ? Early though it occurs, 

 it can scarcely be looked upon as other than somatogenic in its 

 nature. The effects, however, may not be distinguishable from 

 those due to the blastogenic causes mentioned. Here, of course, 

 the difficulty of discrimination renders any inferences difficult 

 and dangerous. The second great group of mechanical malfor- 

 mations is obviously entirely somatogenic in its nature ; and the 

 only dubious point connected with it is the difficulty of deter- 



* " The Origin of Double Monstrosity," Journ. of Anat. and Phys. vol. xxiii. 

 p. 390. 



t " Theories of Heredity." An Address, reprinted from the ' Midland Natu- 

 ralist.' Nov. 1889. 



