36 EARLY AMPHIBIAN DEVELOPMENT 



and except in a few cases, where the vegetative pole is almost 

 exactly uppermost, which condition must be expected to lead to 

 special difficulties in the way of rearrangement of the yolk, such in- 

 verted eggs give rise to normal embryos. The cells at what is now 

 the upper pole divide more rapidly than those at the lower pole, 

 regardless of whether they are pigmented or unpigmented, and the 

 dorsal lip of the blastopore appears at the proper level with regard 

 to the vertical axis.^ 



Gravity is therefore responsible for the fact that in many forms 

 the primary egg-axis is brought into a vertical position in normal 

 development, but it is not responsible for the initial formation of 

 the axis ; nor is gravity an essential factor in normal development, 

 for eggs withdrawn from the directive action of gravity by being 

 forced to roll about continually in a clinostat,^ or by being con- 

 stantly disturbed by a stream of air bubbles,^ nevertheless develop 

 into normal embryos. 



The original determination of the egg-axis, therefore, appears to 

 be due to the development of a primary physiological gradient 

 within the oocyte, which finds visible expression in the graded 

 distribution of cytoplasm and yolk. And this in turn appears to be 

 brought about by factors operative in the ovary which are external 

 as regards the oocyte or egg itself. This point is of considerable 

 importance, for it shows that even this first step in differentiation 

 is externally determined, and is not due to an internal factor or 

 factors.* Cases will be met with where the main axis of the future 

 organism is normally not determined until after the egg is laid, and 

 where its direction can be experimentally controlled (p. 60). 



§2 



The next step in differentiation is the acquisition of bilateral sym- 

 metry. Localisation of the future median plane of the organism 

 has been shown to depend mainly upon the point of entry of the 

 sperm. This has been demonstrated experimentally in the frog by 

 making the sperm enter the egg on a selected meridian, either by 

 means of a fine pipette, or by laying a thread against one side of the 

 egg and allowing a drop of liquid containing sperm to creep along 



1 Pfluger, 1883; Born, 1885. - Roux, 1884. 



3 Kathariner, 1901. ^ Child, 1924, p. 133. 



