228 THE HUMAN SPECIES 



together with the organs of the circulatory and genito-urinary 

 systems are derived. 



It is unnecessary to describe in detail the progress of de- 

 velopment in the embryo ; one striking fact alone needs 

 mention, that in man, as in all other Vertebrates, development 

 proceeds to a point when the Miillerian ducts and Wolffian 

 bodies first become differentiated into male or female genera- 

 tive organs. In man the differentiation can be established in 

 the ninth week. I have pointed out, however, that not only 

 in Vertebrates but also in all metazoa, in spite of the final 

 determination of one sex in the individual, still the potenti- 

 alities of the other sex can undoubtedly be recognised, so that 

 all metazoa possess some features of the duality of sex even 

 though this rarely amounts to visible hermaphrodism. 1 My 

 views are supported by Haecker, 2 whose " double-nucleation," 

 or the physiological importance of both the paternal and 

 maternal sex cells, is recognisable not only in the primitive 

 generative cells of the cyclops but also in the sexual and 

 epithelial cells of the higher metazoa, including man. Haecker 

 sums up the question by stating that the coalescence of the 

 male nucleus with the female nucleus is not merely an absorp- 

 tion into a surrender of independence to a partner but " a 

 creation of a dual personality ". It is as though bi-nuclear 

 generative cells were formed in which the two nuclei could 

 remain widely separated, and, if possible, physiologically inde- 

 pendent of one another. 



Phylogenetically, the chief points of resemblance between 

 the development of man and that of the Vertebrates as a whole 

 consist in the formation of the germinal-layers, the gastrula, 

 the chorda dorsalis, the primitive vertebra, the branchial clefts, 

 the five cerebral vesicles (see Fig. 118), the disposition of the 

 eyes, ears and limbs. 



The three higher classes of Vertebrates (reptiles, birds and 

 mammals) are peculiar in the envelopes which enclose the 

 embryo, namely, the amnion and serolemma (chorion), also in 

 the absence of gills- and the appearance of the allantois, the 

 forerunner of the urinary bladder. 



1 Tubingen, F. Pietzeker, 1904. 



Zeitschr.f, Naturwissensch., xxxvii. 



