No. 2.] EARLY DEVELOPMENT OF AMBLVSTOMA. 359 



There often exists the peculiar condition indicated in Fig. 35 

 in which there is a marked separation of the derivatives of the 

 early blastomeres. This at once suggests the beginnings of 

 double embryos ; while this seems very plausible, it is neverthe- 

 less improbable, since double embryos are but rarely observed. 



Brauer ('94) has observed the same separation in the e.gg of 

 the Scorpion, yet has not been able to determine whether the 

 parts later unite or remain separated and form double embryos. 



Another curious fact is the existence of extra-ovates which 

 keep pace with the segmentation of the blastodisc and show 

 no connection with it whatever. They appear to originate 

 from the layer of cells extending over the yolk, which was 

 presumably in an earlier phylogenetic stage covered by the 

 blastodisc. 



6. General Considerations. 



Polarity. — One of the most striking features of the Am- 

 phibian ovum is its tendency to orient in such a manner that 

 the darker hemisphere is uppermost. Von Baer in 1834 called 

 attention to this difference between the upper and lower hemi- 

 spheres of the Qgg, and by it determined the primary axis of 

 the ovum. The distribution of pigment, the formation of polar 

 globules, the initiation of cleavage, and the accompanying 

 " Faltenkranzen " serve further to emphasize this polar dif- 

 ferentiation. 



Among those who first called attention to this phenomenon 

 may be mentioned Auerbach, Balfour, and Lankester. 



Hatschek ('77) states: " Es ist wahrscheinlich, dass eine 

 polare Differenzirung schon an der ungefurchten Eizelle bei 

 alien Metazoen statt hat, durch welche der vegetative und 

 animale Keimpol bestimmt ist." 



Whitman ('78) observed a marked polarity in the egg of 

 Clepsine, and emphasized its importance. 



Hallez ('85) has noted its occurrence in insect ova, and says : 

 •* La cellule-ceuf possede la meme orientation que I'organisme 

 maternel qui I'a produite ; elle a un pole c^phalique et un 

 p61e caudal, un c6t6 droit et un cote gauche, une face dorsale 

 et une face ventrale." 



