iv GONAD 271 



nucleoli, do not seem sufficiently definite characters as these are 

 probably directly related to volume and metabolic activity of the 

 cell. Cytoplasmic features — of which much use is made in this con- 

 nexion — such as richness in yolk or roundness in shape are also 

 unreliable. As regards the first of these, the study of the develop- 

 ment of embryos rich in yolk brings out clearly the fact that the cells 

 in particular tissues do not, by any means, all keep pace with one 

 another in their developmental processes. Individual cells lag 

 behind, and one of the commonest characteristics of such cells is that 

 the yolk stored up in their cytoplasm remains unaffected for some 

 time after that in the neighbouring cells has been completely used 

 up. Obviously in such a case richness in yolk, even when occurring 

 along with greater size due to less active division, does not constitute 

 evidence of any weight as regards difference in morphological nature 

 between the heavily yolked cell and those round about it. Again 

 there is reason to believe that yolk may be stored up secondarily in 

 particular cells or portions of tissue of a developing embryo as a 

 preparation for future needs quite apart from the actual germ-cells. 



As regards approximation to a spherical shape, it should be 

 remembered that there is a usual tendency for irregularly shaped or 

 branching cells, such> as those of ordinary mesenchyme, to assume 

 temporarily a rounded form at the period during and about mitosis. 

 Such cells are apt to assume an appearance misleadingly like that 

 of young germ-cells. 



The various features above indicated occurring together are 

 sufficient to give a characteristic appearance to the cells in the main 

 gonad but they form hardly definite enough criteria to prove that cells 

 elsewhere are germ-cells in face of the strong probability that the 

 whole mass of germ-cells in the body are of a common origin.- 



Genital Eidge and Genital Fold. — The genital ridge was 

 left as a slight bulging inwards of the peritoneal epithelium covering 

 in the gonocytes. As development goes on the ridge becomes con- 

 verted into a prominent fold — the genital fold. The peritoneal 

 epithelium at first passes continuously over the surface of the strand 

 of gonocytes but soon a change comes about in their relative positions 

 the gonocytes coming to be incorporated in -the thickness of the 

 epithelium which may now therefore be spoken of as germinal 

 epithelium. The gonocytes are to be seen first along the free edge 

 of the fold (Fig. 141, A) and this during subsequent development 

 swells out greatly and forms the functional ovary or testis, while the 

 proximal portion acts merely for suspensory purposes. The gonocytes 

 increase in number by mitotic division but are also reinforced from 

 small apparently indifferent cells lying between them (Fig. 141, C, gc l ). 

 We may take it that these small cells are in all probability to be 

 interpreted as cells of the original gonad which have lagged behind 

 in development, though it is naturally difficult from mere observation 

 to make certain that they are not ordinary peritoneal cells. At 

 a particular stage in development (between 26 and 33 mm. in 



