78 



UNEQ UA L SEGMENT A TION. 



bulky, and where therefore the protoplasm is least concentrated, are 

 larger than the remaining segments, and their formation proceeds 

 more slowly. 



Though where much food-yolk is present it is generally 

 distributed unequally, yet there are many cases in which it is not 

 possible to notice this very distinctly. In most of these cases 

 the segmentation is all the same unequal, and it is probable that 

 they form apparent rather than real exceptions to the law laid down 

 above. Although before segmentation the protoplasm may be 

 uniformly distributed, yet in many instances, e.g. Mollusca. Vermes, 

 etc., during or at the commencement of segmentation the protoplasm 

 becomes aggregated at one pole, and one of the segments formed 

 consists of clear protoplasm, all the food-yolk being contained in the 

 other and larger segment. 



Unequal Segmentation. The type of segmentation I now 

 proceed to describe has been called by Haeckel (No. 105) 'unequal 

 segmentation', a term which may conveniently be adopted. I 

 commence by describing it as it occurs in the well known and typical 

 instance of the Frog 1 . 



The ripe ovum of the common Frog and of most other tailless 

 Amphibians presents the following structure. One half appears 

 black and the other white. The former I shall call the upper 

 pole, the latter the lower. The ovum is composed of protoplasm con- 



Fir,. 40. SEGMENTATION OF COMMON FROG. EANA TEMPOEARIA. (Copied from Ecker. ) 

 The numbers above the figures refer to the number of segments at the stage figured. 



taining in suspension numerous yolk spherules. The largest of these 

 are situated at the lower pole, the smaller ones at the upper pole, and 

 the smallest of all in the peripheral layer of the upper pole, in which 

 also pigment is scattered and causes the black colour visible from 

 the surface. 



The first formed furrow is a vertical furrow. It commences 

 in the upper half of the ovum, through which it extends rapidly, and 

 then more slowly through the lower. As soon as the first furrow has 

 extended through the egg, and the two halves have become separated 



1 Vid<> Eemak, Entwicklunri d. Wirbelthiere ; and Go'tte. Entwicklun.q <L Unke. 



