IV. i INITIAL STRUCTURE OF THE GERM 247 



the isolated parts would be inconceivable. Although the forma- 

 tion of the larval and elementary organs (germ-layers) may de- 

 pend upon such stuffs, yet within each such elementary organ 

 the parts are at first and remain for a time equipotential. 



Differentiation is progressive. The origin of the primary 

 differentiations has been found ; it remains for the experi- 

 ments of the future to discover how fresh distinctions arise 

 until the organization of the whole is complete. Suggestions 

 as to the importance, in this respect, of certain other internal 

 factors have been made, notably of the structure of the nucleus, 

 and the reactions of the parts on one another. These sugges- 

 tions, for they are hardly more than that at present, we shall 

 have to discuss. First, however, we must briefly consider the 

 part played by the spermatozoon in determining the structure of 

 the egg and the symmetry of the embryo. 



12. 



It is of the greatest interest to observe that the definitive 

 distribution of these specific substances in the unsegmented egg, 

 and so the determination of the embryonic axes, may be brought 

 about, partly at least, by the spermatozoon. 



In the Frog (liana fusca, R. temporaria, and P. pains tris, pro- 

 bably also R. escidenta) the egg soon after fertilization becomes 

 bilaterally symmetrical. As first Roux and later Schulze have 

 shown, this is due to the appearance on one side of the border of 

 the pigmented area of a grey crescent, caused by the retreat of the 

 pigment into the interior. The grey crescent subsequently becomes 

 white and added to the white area. The side on which it appears 

 is opposite to that on which the spermatozoon had entered, and later 

 becomes the dorsal side of the embryo ; there is also a considerable 

 correlation between the plane of symmetry thus bestowed upon the 

 egg and the future sagittal plane, since the latter lies in a large 

 majority of cases in or close to the middle point of the crescent. 



This relation between the point of entry of the sperm and 

 the egg- and embryonic symmetry has also been experimentally 

 demonstrated by Roux, who, by applying the sperm to any 

 arbitrarily selected meridian of the egg (by means of a fine 

 cannulus, a brush, or a silk thread), was able to show that 



