212 MR GEORGE BROOK ON THE 



they are produced by the hardening reagents used in preserving the ova. Such, 

 however, is not the explanation. It is in the living egg that such an appear- 

 ance is seen (see fig. 9) before any reagents have been used. There are no 

 such lacunae to be found in sections of hardened material. The appearance, I 

 think, admits of a similar explanation to that which has been given for the 

 "vacuoles," of which it is supposed to be the remnant. The base of the ger- 

 minal disc is not flat, as was supposed by Kupffer. There are proceeding 

 from it a number of broad but tapering strands of protoplasm, which form the 

 means of communication between the germinal disc and the yolk. In this 

 region the mixture of yolk and protoplasm is more transparent than the more 

 solid yolk mass, and thus in optical section appears as a cavity. The peculiar 

 shape of these so-called lacunae appears to be altered by hardening agents, as 

 I have never in section met with such flask-shaped masses as are seen in 

 the living egg. Later, as the protoplasmic filaments are withdrawn, the central 

 mass of yolk spheres lose their rounded outline, and appear to fuse together; 

 whereas the more peripheral ones retain their primitive form, being always 

 more or less surrounded with protoplasm. Thus, again, in these later stages 

 the central portion appears more transparent than the peripheral zone. 

 Kupffer's interpretation also requires modification in another respect. One 

 is led to conclude from his remarks that the yolk is directly transformed into 

 protoplasm in the " transition area " already spoken of. This is not really the 

 case. The yolk only becomes available for the use of the blastoderm after it 

 has been assimilated and digested by the existing protoplasm. While the 

 germinal disc is in progress of formation, its somewhat conical and ill-defined 

 base is actively engaged in this process of assimilation. In the base of the 

 disc large masses of food yolk may be seen entangled between the branching 

 filaments, and these get smaller and smaller as they are pushed farther away 

 from the active area. 



It is important to note that at the time the upper portion of the disc is 

 ready for segmentation the lower portion is still actively fulfilling a vegetative 

 function. 



It thus comes about that both Kupffer and Hoffmann were partly correct 

 in their conceptions of the germinal disc, but, according to my view, neither of 

 them were entirely so. It is by a union of both ideas that a true compre- 

 hension of the question is obtained. 



The egg of the herring has now reached a stage when it is comparable with 

 those of other fishes. Indeed, so nearly does it approach to Waldeyer's (26) 

 ideal of a meroblastic ovum that it might very well have served as his type. 

 Before the first furrow appears the egg is made up as follows : — 



1. Of a large collection of protoplasm in the germinal area in which segmen- 

 tation subsequently commences. 



