220 MR GEORGE BROOK ON THE 



lated in the same manner in the herring as it is in the amphibian ovum. In 

 the herring the food yolk must be digested before it can be available as nourish- 

 ment for the embryo. Had the separation of protoplasm from yolk been com- 

 plete this end could not have been accomplished. It is therefore the function 

 of the residual protoplasm in the yolk pole to make the food supply available for 

 the use of the embryo. This is accomplished by a process which is essentially 

 one of intracellular digestion, since the protoplasm in the yolk pole must be 

 regarded as having the value of a cell, and is directly derived from the germinal 

 area. The cortical protoplasm incorporates a portion of the yolk material 

 within its substance, digests it, and thus adds to its bulk. A store of available 

 material is thus laid up, which is utilised as occasion requires. It thus appears 

 that the food yolk of the herring ovum is more nearly comparable in its manner 

 of assimilation with the albuminous food supply of Lumbricus and some insects, 

 than with the food yolk of the amphibian ovum. Nevertheless the yolk pole 

 in the herring ovum has the same morphological value as that of the amphi- 

 bian ovum, and it is mainly owing to the difference in distribution of the 

 constituent parts that their subsequent behaviour is not identical. How far 

 the tissues derived from each pole are identical in the two cases will be 

 considered later. 



Although I am firmly convinced that in the herring ovum the third furrow 

 is an equatorial one, I am not at present prepared to assert that this is the case 

 m all fish ova. Nevertheless, it appears to me probable that an equatorial 

 furrow will ultimately be shown to exist in all Teleostean ova at an earlier stage 

 than has generally been supposed. 



To take the case of pelagic ova. In a paper on the development of Trachinus 

 vipera (5) I have described the first furrow, which takes an equatorial direc- 

 tion, to be the fifth of the series, and to be formed in the four central cells of 

 the 16-cell stage when the 32-cell stage is being produced. I am now, however, 

 inclined to think that I have neglected to observe one furrow altogether — that, 

 namely, which divides archiblast from parablast. At the time the ovum is 

 fertilised the yolk consists of one large yolk sphere and not of a number of 

 small ones, as is the case in the herring ovum. There is also no appreciable 

 increase in the quantity of germinal protoplasm after the egg is fertilised. The 

 germinal protoplasm in the ripe unfertilised ovum consists of an even layer 

 entirely surrounding the yolk. As the circumference of the yolk sphere is 

 quite smooth, the line of demarcation between protoplasm and yolk is well 

 marked. After fertilisation the bulk of the germinal protoplasm sinks to the 

 lower pole of the ovum to form the germinal disc. A thin film of protoplasm 

 is, however, still left surrounding the yolk, and this gets thicker towards the 

 germinal area. The first furrow appears as a longitudinal depression across the 

 centre of the circular disc, and is pushed down towards the yolk. A little later 



