254 THE MOSAIC STAGE OF DIFFERENTIATION 



similar movements. But, for this smaller embryo, these movements 

 result in the folding and eventual fusion of its sides ventrally instead 

 of dorsally, since it is back-to-back with the larger embryo. Thus 

 the smaller embryo is inside-out : its limbs are contained in a closed 

 cavity lined by its body- wall which is completely inverted; its 

 organs and viscera lie outside its body-wall, and in contact with 

 those of its larger brother, inside which it is.^ In spite of these 

 form-changes being the reverse of normal, histological differentia- 

 tion continues as if nothing had happened (fig. 122). 



While "dynamic determination", or the determinative eifects of 

 form-changes, may possibly be operative in the case of organisers 

 (see Chap, vi, p. 163), it does not seem that histological differentia- 

 tion in the mosaic stage of development is dependent on it. 



§11 



A special section may be devoted to the problem of the gonads 

 and sex-differentiation, which present many interesting features. 

 A full discussion of all aspects of the question has been given in 

 recent books such as The Development of Sex in Vertebrates'^ and 

 Sex and Internal Secretions ;^ accordingly here much controversial 

 detail will be omitted. Here, only such points as bear upon morpho- 

 genesis and the problem of differentiation will be dealt with, and 

 they in broad outline. 



In general, the vertebrate gonad arises as what is doubtless a 

 special gonad-field on the dorsal side of the coelom. It first consists 

 of thickened coelomic epithelium (germinal epithelium) with some 

 underlying mesenchyme, together with primordial germ-cells. In 

 many vertebrates, these are undoubtedly differentiated precociously, 

 in most cases in the endoderm, and then migrate into the gonad- 

 rudiment. In other cases, especially among the higher forms, it 

 seems equally clear that germ-cells arise directly from the germinal 

 epithelium. It is possible that both these sources contribute to the 

 formation of germ-cells in many vertebrates. 



Later, the gonad-rudiment becomes differentiated into an ex- 

 ternal cortex and a central medulla, but the details vary considerably 

 in different groups. 



1 Seidel, 1929. - Brambell, 1930. 



2 E. Allen, 1932. 



