A DESCRIPTIVE SKETCH 15 



As a rule among Amphibia, the ist polar body is given off before 

 fertihsation, and the 2nd polar body after that event. The fertilisa- 

 tion of the egg by the sperm has a threefold significance. In the 

 first place it activates the egg to begin its development; secondly, 

 it brings in to the resulting zygote its supply of paternal hereditary 

 factors ; and lastly, it is responsible for bringing about the next step 

 in differentiation, which is the determination of a plane of bilateral 

 symmetry. 



In the frog it has been shown by experiment that the mid-ventral 

 line of the embryo will be formed close to the meridian on which the 

 sperm enters the egg.^ The only visible differentiation at this stage, 

 however, concerns the dorsal side, opposite the point of sperm 

 entry. A region of this, rather below the equator of the egg, is 

 marked soon after fertilisation by changes in the surface layer, lead- 

 ing in the case of the Anura to the formation of the so-called grey 

 crescent, due to the retreat of pigment into the interior of the egg. 

 Analogous, but less well-marked changes on the dorsal side of the 

 recently fertilised egg are observable in the Urodela.^ 



After the entry of the sperm, therefore, the developing organism, 

 although still a spherical object, has all three of its axes determined. 

 The antero-posterior axis and the dorso-ventral axis of the future 

 embryo lie in the plane of bilateral symmetry, which, in turn, 

 passes through the original egg-axis of polarity. At the same time, 

 the transverse, or left-right axis, is also necessarily fixed with the 

 determination of the other two axes. The symmetry relations of 

 the organism are thus completely and definitely fixed (fig. 2). 



The grey crescent of the Anuran egg (or its equivalent in the egg 

 of Urodela) is the place at which the next marked step in differen- 

 tiation appears. The egg has meanwhile undergone cleavage, and 

 instead of being a single large cell, has come to consist of a large 

 number (over a thousand) of smaller cells or blastomeres, which 

 enclose a small cavity, the blastocoel. These blastomeres are smaller 

 in the animal hemisphere than in the vegetative. This is a result of 

 the prime differentiation of polarity, for yolk retards cell-division, 



^ Roux, 1903; Jenkinson, 1907, 1909 A. 

 - Vogt, 1926 B. 



