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A specially instructive case in this connexion is that of Dinophilus, a worm which produces two kinds of eggs-large female eggs and small male eggs. In spermatogenesis, sperma­ tids of one kind are formed, each of which receives altogether 10 chromosomes (the haploid number). The ovary of the justhatched female consists of a few oogonia, which multiply by normal nuclear division. After the first growth-period all the eggs are of the same size. Insemination takes place during the third growth-period-before the reserve materials of the egg have been formed, but after the differentiation into male and female eggs which involves sex-determination. Insemi­ nation and fertilization are therefore without influence upon sex-determination, and there is no question of alternative combinations of the chromosome apparatus being responsible. Closer investigation of the maturation divisions confirms this. Maturation is only completed in the egg after it is laid. Maturation and fertilization take place in all the eggs in exactly the same way. In the first maturation division 10 tetrads appear; 10 chromosomes remain in the egg; each fertilized egg has 20 chromosomes, the diploid number. No heterosome apparatus,

differing according to the sex, can be demonstrated. The nuclei of the two kinds of eggs are distinguishable only by their size ; any morphological cause for the sex-differentiation of the eggs is entirely lacking. Sex is already decided in the growing oocytes, i.e. in the unripe eggs; they have a male or a female constitution. Nothing definite can be said about the internal processes which here produce sex-determination, but we shall certainly not be far wrong in not referring this determination to a chromosome mechanism acting as the heterosome apparatus does when that is present.