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[38]. [38] _Vide_ Remak, _Entwicklung d. Wirbelthiere_; and Götte, _Entwicklung d. Unke_. 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 containing 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. [FIG. 40. SEGMENTATION OF COMMON FROG. RANA TEMPORARIA. (Copied from Ecker.) The numbers above the figures refer to the number of segments at the stage figured.] 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 from each other, a second vertical furrow appears at right angles to the first and behaves in the same way (fig. 40, 4).