5o8 



EVOLUTION, HEREDITY, EUGENICS 



I, 2, 4, 8, etc., the germinal substance is transformed by " cleavage " 

 into the new organism.^ 



Cleavage and Organ Formation.- 



C^ P 



4.-'*' -;2^-N 



^-c=» §=»— / 



Gametes 



Fig. 262. Reduction Division In Relation 

 to Sex Determination. (After Gruenberg, Evolu- 

 tion, published by D. Van Nostrand Co., Inc.) 



The chromosomes of the male (light) parent 

 (P) differ from those of the female parent (dark) 

 in the form of the Y chromosome. As gametes 

 are formed, one of each pair of the X chromo- 

 somes (black) goes to each egg. In the forma- 

 tion of sperms, however, half get the X chromo- 

 some and half get the Y chromosome: there is 

 only one kind of egg, but there are two kinds of 

 sperms. In the following generation, Fi, the 

 sperms containing the X chromosomes give rise 

 to females, and those containing the Y chromo- 

 somes give rise to males. Both males and 

 females of this generation, however, derive half 

 the chromosomes from the father and half from 

 the mother. 



^ Consult the paper by H. B. Goodrich, 1929, 

 Rev. of Biology, vol. 4, no. i, pp. 83-99, March. 



-Subsequent to fertilization 

 certain changes take place 

 converting the egg cells 

 into cells that multiply and 

 form the embryo. Cleav- 

 age may be cotnpkfe, and 

 equa/ as in Amphioxus or 

 unequal as in the frog. 

 On the other hand it may 

 be partial, and either super- 

 ficial as in the eggs of Arth- 

 ropods or discoidal as in the 

 chick. 



One of the most fascin- 

 ating studies imaginable is 

 that of organ formation 

 from the primitive embry- 

 onic state. It is possible 

 to follow the development 

 of some invertebrate types 

 such as worms and molluscs 

 from the one-cell stage to 

 the actual development of 

 complete organs. Their 

 condition of semi- trans- 

 parency and the distinctive 

 colors of certain developing 

 organs render this task 

 quite easy. The nervous 

 system has been traced to 

 the two-cell stage in Cre- 

 pidula, a mollusc, by Dr. 

 E. G. Conklin. 



Germinal Layers. — 

 At the end of cleavage 

 (segmentation) the egg is 



Mendelian inheritance in fish. Qu. 



