188 T HE ANCESTRY OF VERTEBRATES ^ 



Displacement of the endoderm-area. — We shall see now 

 what we find of these phenomena in the frog egg, and 

 take as an example the egg of Rana fiisca. At once it 

 will be evident that the first of the three above-mentioned 

 processes, the wandering of the endoderm area to the ventral 

 (in Chordates to the dorsal) side, is here peifoimed very 

 precociously, immediately a*"ter fertilization, and conse- 

 quently is already finished in the unsegmented fertilized 

 egg. Thus we find, as is shown by fig. 35, that the white 

 area here does not lie diametrically opposite the animal 

 pole but much more to the future dorsal side which cor- 

 responds to the ventral side of the Annelid. The boundary 

 between the ecto- and endoderm areas probably runs parallel 

 to the demarcation of the darker and lighter areas of the 

 egg, as may also be concluded from the place where after- 

 wards the dorsal and ventral borders of the blastopore appear 

 (fig. 35). The same displacement of the endoderm area which 

 in the Annelid occurs during development is evidently 

 alreaiy completed in Chordates in the unsegmented egg. 

 However, in Annelids also we often find such a structure 

 of the egg, as may be concluded from the relative size of 

 the cleavage cells. In my article on the development of 

 Scoloplos (1916) 1 have tried to show that among the eggs 

 of polychaetous Annelids three types are to be distinguish- 

 ed. In the first place we have the small, poorly yolked, 

 eggs of Polygordius, Hydroides etc., in which the cleavage 

 results in a very equal coeloblastula (fig. 38a). Secondly 

 we have the larger eggs of other species in which two 

 types of polarity may be very early recognized, which exert 

 their influence on the very determinate cleavage. In the first 

 place we can distinguish the polar or radially symmetrical 

 polarity, manifesting itself in the accumulation of yolk at the 

 vegetative pole which again causes the endoderm cells to be 

 much larger than the cells of the three quartets of ectomeres, 

 so that, in fact, they deserve the name macromeres. In the 

 second place the bilateral polarity which manifests itself in 

 the cells of the rear side (cf-side) being from the beginning 

 much larger than the corresponding cells at the anterior 

 side (6-side), so that the entoderm area from the beginning 

 does not lie diametrically opposite the animal pole. The 

 diagrams of fig 386 and c may serve to illustrate this. 

 As a rule we see in the eggs with a larger diameter both kinds 

 of polarity exerting their influence on the cleavage at the 



