36 



Appendices to Fourth Annual Report 



for a short time, and the collection at the yolk pole is sometimes so great 

 that it may easily be mistaken for the true germinal area. 



Segmentation now begins ; but while it is in progress the portion of 

 protoplasm situated in the base of the germinal mound, and including 

 the processes pushed in amongst the yolk-spheres, continues its digestive 

 function, and, indeed, this process goes on without intermission until the 

 whole of the food-yolk has been absorbed. The first furrow is a vertical 

 one (meridianal). The second is also meridianal in direction, but in a 

 plane at right angles to the first. Neither furrow is, however, pushed 

 down to the base of the germinal mound, as seen in optical section. 

 From what has been said, it will be seen that there is no well-defined 

 limit between protoplasm and yolk, but that the basal portion of the 

 germinal area is a mixture of protoplasm and yolk which is undergoing 

 assimilation. 



In the living egg a very interesting phenomenon may frequently be 

 observed, which throws considerable light on the early segmentation 

 stages. It often happens that shortly before the formation of the first 

 furrow a collection of protoplasm is to be noticed at the yolk-pole. As 

 soon as the first traces of a furrow are to be seen this protoplasm sinks, 

 leaves the yolk-pole, and joins that in the germinal area. After the first 

 furrow has been pushed down as far as the intermediate digestive area, 

 a portion of the protoplasm sinks back to the yolk-pole, only to leave it 

 again on the commencement of the second furrow. After the formation 

 of the second furrow a collection of germinal protoplasm is again to be 

 noticed at the yolk-pole. 



The appearances here described are not abnormal ; the process has been 

 already observed by Kupffer, who suggested the term gegenhugel for the 

 collection of protoplasm at the yolk-pole. 



In the herring the third furrow takes an equatorial direction. Prior 

 to its formation the protoplasm at the yolk-pole is included in that of 

 the germinal mound, and of necessity the protoplasm remaining in the 

 lower pole is no longer in communication with the segmenting disc. 

 After the formation of the two first furrows the germinal mound was 

 divided into four segments, whose bases were not defined. The third 

 furrow simply completes the bases of these four segmentation spheres, so 

 that they are then shut ofi" from connection with the cortical protoplasm 

 and the yolk. The upper pole is the germinal disc, which goes on 

 segmenting ; it constitutes the ArcMblast of Klein (4).''^ The lower pole 

 consists of a central mass of yolk, around which is a cortical layer of 

 protoplasm, which is the Parahlast. 



Nothing has hitherto been said as to the behaviour of the nucleus 

 during these early stages of segmentation. As a matter of fact, I have 

 not been able to find a nucleus nor any trace of nuclear division either in 

 the living egg or in sections of prepared material, I have been led, 

 therefore, to conclude that the nuclear material is not collected into a 

 definite recognisable mass in these earliest stages of division, but is 

 generally distributed throughout the germinal protoplasm. The behaviour 

 of the germinal vesicle as the egg increases in size appears to support 

 this view. After the completion of the base of the first four segmenta- 

 tion spheres — that is, after the formation of the first equatorial furrow — 

 a nucleus appears in each of the archiblast cells. This has the appearance 

 of a transparent vacuole-like spot toward the centre of each segmentation 

 sphere, and always disappears before the commencement of each succeed- 

 ing furrow, to reappear again after the division is complete. 



* The term Archiblast was originally suggested by His ; it is however used here in 

 a more modified sense than was extended to it by that author. 



