190 Proceedings of the Royal Physical Society. 



Turning now to the consideration of holoblastic ova, let us 

 glance at the structure of the ovum of the Mollusca and the 

 Amphibia. In the unsegmented Qgg there is no separation 

 of protoplasm from the yolk. Even after the formation of 

 the first equatorial furrow, when the ovum is divided into an 

 animal and a vegetative pole, it can only safely be stated 

 that the animal pole is richer in protoplasm, while the vege- 

 tative is richer in food-yolk. The food-yolk being distributed 

 throughout the protoplasm with each succeeding division of 

 the Qgg, cells are produced, which consist partly of protoplasm 

 and partly of yolk. Each cell, in fact, carries its food supply 

 along with it. There is thus no special store of nutriment 

 in a holoblastic ovum such as is found in a meroblastic one. 

 A meroblastic ovum is generally regarded as an ovum which 

 is only prevented from segmenting as a whole by the presence 

 of a preponderating amount of food-yolk. This undoubtedly 

 expresses a part of the difference between, say, a Teleostean 

 and an amphibian ovum, but 'not, as I take it, the charac- 

 teristic difference. I can conceive of an ovum containing 

 precisely the same proportion of food-yolk to protoplasm as 

 is found, say, in the egg of the frog, but which would not be 

 holoblastic in its segmentation. If, for instance, the whole 

 of the yolk material in the frog ovum wxre concentrated 

 into a solid yolk mass, instead of being distributed throughout 

 the germinal protoplasm, complete segmentation could not 

 take place. Before a furrow can be pushed down through a 

 given area, it is essential that a certain amount of protoplasm 

 should exist there. The rapidity of segmentation in any part 

 of an ovum is directly proportional to the concentration of 

 protoplasm in that part, and where there is practically no 

 protoplasm, it is impossible for segmentation to progress. 

 The difference in the distrilution of protoplasm and food-yolk 

 is therefore to be regarded as an essential feature of a mero- 

 blastic Qgg. 



It is this difference in distribution of the food-yolk which 

 necessitates a different mode of assimilation. Knowing that 

 in the amphibian ovum the first furrow progresses more slowly 

 in the vegetative than in the animal pole, and that this is 

 occasioned by the greater amount of food material in the 



