ARTICLES 397 



viving two cells form a half-tadpole, and the same is true of the 

 egg of a Ctenophore from m which a half- larva can be produced. 

 This seems to fit in beautifully with the picture-puzzle theory of 

 development, but what are we to say of the further perplexing 

 fact that, if the half-tadpole or half-larva remains alive, it will 

 " post-generate " the missing half ? But worse blows than even 

 this have been rained on the theory by the progress of experi- 

 mental embryology. The eye of a vertebrate (leaving out lesser 

 detail) consists of two essential parts, the lens and the retina. 

 The lens is formed from the skin of the side of the head, but 

 the retina originates from the brain. If, now, the skin of a young 

 tadpole be slit open, and the earliest rudiment of the retina be 

 cut off from the brain and pushed back under the skin till it 

 attains the level of the shoulder, or even farther back, the rudi- 

 ment will continue to live and grow in its new position, and 

 will compel the skin covering it to form a lens, although this skin 

 has never in the history of the race formed a lens before. We 

 are really entitled to ask whether this area of skin acquired the 

 " determinants "for a lens in anticipation of the enterprise of 

 a biological investigator ? Or, again, if the lens of a newt's eye 

 be cut out, the edge of the retina will bud off a new lens. How 

 is the animal able to foresee this experiment and repair the 

 damage that results ? Finally, although the diligent search of 

 Weismann's pupils has enabled them to detect the first traces of 

 germ-cells at an early stage in the development of Vertebrates, 

 yet later investigations have shown that, although in the frog, 

 for instance, these early-formed germ-cells are undoubtedly 

 present, they are not numerous enough to account for the eggs 

 that a female frog lays during its first breeding season, and that 

 many of these, and all those laid in subsequent seasons, are 

 developed from the modification of ordinary tissue-cells ! In 

 the white mouse the early-formed germ-cells are all absorbed, 

 and all the viable eggs are produced from peritoneal cells. In 

 fact, all the trend of modern investigation is to show that all the 

 nuclei produced by the division of the egg-nucleus are potenti- 

 ally alike, and that what undergoes differentiation is not the 

 nucleus, but the cytoplasm. 



But, although the detailed scheme of the germ-plasrn as 

 put forward by Weismann has proved to be untenable, the idea 

 of the unchangeable character of the hereditary potentialities 

 of the egg has taken a firm hold on the minds of Mendelian 

 investigators. As Punnet phrases it, " The creature is not 

 made, but born." What Mendelians have shown is that when 

 a " mutant," which almost always differs from the type in the 

 loss of some feature, is crossed with the type, a hybrid is pro- 

 duced which gives rise to two kinds of germ-cells, one like the 

 mutant and one like the type. They have further put forward 



