478 GENERAL CONSIDERATIONS 



regation, producing a purity of the germ cells. The unit chaiacters are 

 external features, the hypothecated determiners for these are provision- 

 ally located in the chromosomes. 



Sex is of course inherited in the sense that an individual resembles 

 either its father or its mother in respect of sex. Moreover, considering a 

 species as a group, the two sexes are approximately numerically equal. 

 This is the ratio expected when a Mendelian pure recessive (homozygote) 

 is crossed with a hybrid dominant (heterozygote) for again by the laws of 

 chance, the chances are equal that a determiner shall meet with its oppo- 

 site or its like. On the basis of these considerations mainly, coupled with 

 the data of sex-linked inheritance (see Morgan, "Heredity and Sex." 1915), 

 sex is conceived by some to be inherited in Mendelian fashion, the indi- 

 vidual with a single accessory (male) being regarded as the heterozygote, 

 that is, digametic; that with two accessories, the homozygote, that is, the 

 homogametic. However, such serious contradictions and difficulties arise 

 when the matter is thus strictly interpreted that a simple Mendelian view 

 of sex ratios must probably be abandoned. In any event the interpretation 

 that regards the heterochromosome, when present in the duplex condition, 

 as in the female, as an inhibitor to male sex development from- the viewpoint 

 of secondary sex characters, accords well with much of the experimental 

 evidence from vertebrates. The bulk of the best evidence tends to show 

 that sex is determined at fertilization, and that it cannot thereafter be 

 altered by control of environmental conditions. Relatively much chro- 

 matin may be thought to underlie the anabolic condition characteristic of 

 femaleness, relatively less chromatin the katabolic conditions which sub- 

 serve the high differentiations of maleness. 



Oogenesis. We must now return to a detailed description of the 

 oogenesis. Splendid material for the presentation of the facts con- 

 cerning this process is the egg of Ascaris, or that of certain molluscs, 

 or the more easily available egg of some echinoderm, e.g., starfish. This 

 description will confine itself to the starfish egg. The essential points 

 can be well illustrated with this material (Plate D, Figs. 1-16). In con- 

 trast to the spermatozoon, which is relatively minute, highly motile, and 

 with a minimum amount of 'cytoplasm, the egg on the other hand grows 

 enormously in size (ratio of sperm to egg 1 : 500,000 ), is non-motile, 

 and has a large amount of cytoplasm heavily laden with yolk material. 

 This phase of the oogenesis is known as the growth period; during the 

 early portions of this period synapsis occurs, and subsequently chro- 

 matic material (chromidia) appears in the cytoplasm, presumably ex- 

 truded by the nucleus, and in part at least changed into yolk. This 

 generation is the primary oocyte, the preceding generation being 



