GENERAL EMBRYOLOGY 141 



In this scheme each character appears once from the father, once from the 

 mother, the result being sixteen combinations, of which nine are blue and smooth, 

 three are blue and wrinkled, three white and smooth, and one white and wrinkled. 

 Of these four groups only the last (-^ ) is homozygote with reference to both 

 pairlings. Of the other groups there is only one homozygote in each, BS, 

 BR, and WS, iV of each. Of these four homozygote, true breeding forms, 

 two, BR and WS are reversions to the beginning type; the other two, BS and 

 WR, are new and constant associations. They are the 'analytic species' to 

 which allusion was made on an earlier page, which have no new characters 

 but only a new combination of characters. Of the other, not true breeding 

 forms, two IFs (g of the whole) as to color, w, and two bR, as to shape, are 

 homozygote. Among the blue forms there is the greatest variety: two bS are 

 constant in shape, two Bs constant in color, four bs are heterozygotes in shape 

 and color and are capable of splitting. 



It is evident that with every new character entering, the number of com- 

 binations possible increases enormously, as does the number of new analytic 

 species. Therefore in high polyhybrids many thousands of descendants are 

 necessary to have all possible combinations actually realized. It is also apparent 

 that species with a low rate of reproduction many combinations may not 

 appear, others only in distant descendants (atavism). 



The numerical relations 9:7 (nine blue-smooth to seven of all others) is 

 important, since it is characteristic of the second generation of dihybrids. Where 

 it occurs it indicates that two pairlings are concerned, even when other facts seem 

 to point to the presence of but one. Thus, in the crossing of white breeds of 

 fowl, only colored animals appear in the second generation, but in the grand- 

 children there are nine colored, seven white. So one can say, with certainty, 

 that the color depends upon the mutual action of two substances, which must 

 be present simultaneously and as dominants. If these conditions be not ful- 

 filled, the color is lacking and is replaced by white. 



(4) Mendel's law is chiefly concerned with the crossing of varieties; it 

 appears to have no significance for species, since with them there usually appear 

 varying degrees of intermediate forms which remain constant in the m-xt 

 generation. But this cannot be proved in most cases, since hybrids are 

 usually infertile. Still it does not appear that a fundamental distinction exists 

 between species and varieties. Farther researches are necessary. 



The great significance of the results of Mendel and his successors is apparent. 

 There has been established a wide reaching harmony between the theoretical 

 conclusions based upon the phenomena of maturation and fertilization and 

 facts obtained by crucial experiments. Further, the extraordinarily complex 

 phenomena of inheritance are now capable of expression in exact mathematical 

 form. The knowledge of this regularity of inheritance has an enormous value 

 for practical animal (and plant) breeding, since it affords the foundation for 

 intelligent procedure. 



3. Cleavage Process. 



Arrangement of the Cleavage Planes. The fertilized egg-cell 

 divides in rapid succession into 2, 4, 8, 16, etc., cells, which become con- 

 tinually smaller, since the mass of the egg does not increase. The evils 

 are called cleavage spheres, or blast omeres, the whole process the cleavage 

 process, or segmentation, because, at each division, furrows arise on the 

 surface which continue to penetrate more deeply until finally the cells are 



