62 THE AMPH10XUS. 



opening of the gastrula-motith appears situated some- 

 what in the direction of the dorsal surface. 



The embryo takes further an elongated form, so 

 that, 011 the front view, it appears somewhat oval, and 

 in profile shows a dorsal surface parallel to the longi- 

 tudinal axis and quite flattened out, on the posterior 

 end of which lies the already considerably narrowed 

 gastrula mouth (Figs. 33, 3-1). 



The manner in which these successive stages follow 

 from one another certainly admits of various explana- 

 tions. It may be considered, and this is more or less 

 the view of Kowalevsky, that the axis drawn from the 

 upper to the lower pole corresponds to the later 

 longitudinal axis. This view would farther say that 

 the gastrula-mouth, from quite the beginning (Fig. 

 24), corresponds to the posterior end, that during the 

 diminution of the gastrula-rnouth the extension of 

 the embryo is continually advancing, and that the 

 gastrula-mouth only in the last stages experiences a 

 movement towards the back. I will not absolutely 

 deny the fairness of this view, although I consider, as 

 far more probable and more consistent with the facts, 

 ari explanation which I will now proceed to give. 



This explanation finds its expression in the examina- 



