218 MR GEORGE BROOK ON THE 



united towards the base. I have seen similar effects in the eggs of the 

 Gad'uhv, but these, I think, have been caused by too great an elevation of 

 temperature. In such cases the separation may become complete, when develop- 

 ment is at once arrested, and the egg dies. 



Returning now to the position held by Kupffer, it will, I think, be seen 

 that he must have been mistaken in the order of segmentation. It is only 

 fair to state that his opinion, that the second furrow takes an equatorial 

 direction, was founded on an observation of the process in the pike, and not 

 in the herring. He concludes, however, that the early phases of segmentation 

 are identical in both species. If the second furrow is equatorial in direction, 

 why should the cortical protoplasm flow forwards from the yolk pole to the 

 germinal area when the third furroiv is about to be formed ? If there is an 

 equatorial furrow already in existence, the upper portion of the germinal 

 area which it defines must be cut off from communication with the lower 

 part, and with the yolk. That this is not the case is shown by the fact 

 that after the formation of each of the first three furrows, the cortical proto- 

 plasm diminishes in quantity. There is a further point. The cortical proto- 

 plasm flows from the yolk pole to the germinal area, and becomes a part of 

 the latter during each of the first three segmentation stages. After this the 

 remaining cortical protoplasm, which has very much decreased in bulk, does 

 not again flow towards the germinal disc, until the latter consists of a mulberry 

 mass of cells, and awaits the co-operation of the parablast. The cortical 

 protoplasm continues to exist, and indeed to increase in bulk, but it is 

 evident it can no longer take part in the segmentation of that portion of 

 the germinal area which has been cut off from communication with it by 

 the formation of an equatorial furrow. Nevertheless, it has a very important 

 part to play. 



The same arguments may be brought against the assertion of Hoffmann, 

 that the first furrow takes an equatorial direction in the fish ovum. Hoffmann 

 appears very certain about his interpretation, and gives a figure in which the 

 nucleus is situated towards the base of the germinal disc, and in which the 

 elongation of the nucleus during karyokinetic division takes a vertical direction. 

 The plane of division is therefore at right angles to the nuclear axis, so 

 that an equatorial furrow is formed. 



It would not be just to deny the accuracy of such observations from a 

 study of different material. Hoffmann's investigations were made on Julis, &c, 

 and mine on the herring. It is possible that a difference in species may 

 allow of a difference in the order of development. All I can say is, that 

 in the herring neither the first nor the second furrow takes an equatorial 

 direction, according to my own interpretation of the process. If either of them 

 did, the whole plan of development would, to my mind, be changed. 



