THE SEGMENTATION OF THE OVUM. 95 



yolk : in the vast majority of eggs there is present however a con- 

 siderable bulk of food material usually in the form of highly re- 

 fracting yolk-spherules. These yolk-spherules lie embedded in 

 the protoplasm of the ovum, but are in most instances not distri- 

 buted uniformly, being less closely packed and smaller at one pole 

 of the ovum than elsewhere. Where the yolk-spherules are few- 

 est the active protoplasm is necessarily most concentrated, and 

 we can lay down as a general law 1 that the velocity of segmen- 

 tation in any part of the ovum is roughly speaking proportional 

 to the concentration of the protoplasm there ; and that the size 

 of the segments is inversely proportional to the concentration of 

 the protoplasm. Thus the segments produced from that part of 

 an egg where the yolk-spherules are most 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 dis- 

 tributed 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 proto- 

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

 formed consists of clear protoplasm, all the food-yolk being con- 

 tained in the other and larger segment. 



Unequal Segmentation. The type of segmentation I now 

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

 equal 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 2 . 



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 



1 Vide F. M. Balfour, " Comparison of the early stages of development in Verte- 

 brates." Quart. Join-, of Micr. Science, July, 1875. 



2 Vide Remak, Entwicklnng d, Wirbelthiere; and Gotte, Entwicklung d. Unke. 



