158 



THE ANCESTRY OF VERTEBRATES 



accuracy. In Vertebrates, however, the cleavage is much 

 more indeterminate and the relatively equal size of ihe 

 cleavage-cells and their great number, when differentiation 

 sets in, very soon make it impossible to find out any longer 

 the place of the animal pole, where the first two cleavage- 

 furrows have intersected. Thus we must look for another 

 landmark, or make it ourselves, to enable us to determine 

 the place of the animal pole. The egg of the anchovy 

 (EngrauUs encrasicholus) presents certain advantages in 

 this respectv It is characterized by its oblong shape in 

 which it reminds one of the eggs of Cephalopods. It agrees 



Fig. 33. Three stages of development of the egg of the anchovy 

 (EngrauUs encrasicholus), b and c after Wenckebach, 1886. 

 a morning of the 1st day, b morning of the 2nd day, c 

 evening of the 2"d day. 



Cfi notochord, m micropyle, of auditory vesicle, pol polar 

 bodies. 



with the latter also in that after fertilization the animal 

 pole, with the nucleus and an accumulation of protoplasm, 

 is situated at one of the extremities of the unsegmented 

 egg, right under a little opening in the chorion, the micropyle 

 (Wenckebach, 1887). The first cleavages have not yet 

 been studied, the spawning taking place very early in the 

 morning or in the night. The development takes three days. 

 The eggs fished in the morning of the first day of their 

 development all show a little round germinal disc sitting 

 like a little cap on the animal pole. Above the centre of 

 this disc the micropyle and often also the polar bodies 

 may be seen. In the course of the first day and night this 

 germinal disc extends in a concentric way over the surface 

 of the egg, the circular circumference during this process 



