80 



The fertilised ovum ol' llie sole, then, jjossesses the same stiucture as was above- 

 described in the unfertilised, aud also something not mentioned in the previous 

 description, namely, a nucleus in the germ formed by the coalescence of two pronuclei, 

 one derived from the nucleus of the unfertilised eg^:, the other from the head of a 

 spermatozoon. 



The first visible change which takes place after fertilisation is the division of the 

 germinal mass into a number of small segments. The mass first divides into two 

 halves separated by a superficial furrow extending across its middle. Another furrow 

 then appears crossing the first, so thai the mass is divided into four portions. Each of 

 these divides again into two and then into four, so that there are now sixteen segments. 

 Then divisions take place in a direction parallel to the surface of the mass, so that it 

 comes to consist not of one layer of segments, but of several layers. As this process 

 of segmentation continues the segments continually become smaller, so that the 

 condition is reached which is seen in Plate XV, Fig. 3. During this time movement of 

 the protoplasm on the surface of the yolk and between the yolk segments has caused the 

 latter to extend somewhat beyond the edges of the germinal mass. 



The mass of protoplasmic segments into which the original single large mass has 

 thus been converted now begins to become thinner and broader, extending itself 

 so as to envelop the yolk, us in Plate XV, Fig. 4. When lliis })rocess begins a cavity 

 is formed between the yolk and the central part of tlie germinal mass. The germinal 

 mass as it extends over tlie surface of the yolk soon becomes so thin that it must now 

 be called the germinai membrane (blastoderm). Fig. 5 shows the stage at which the 

 germinal membrane has enveloped more than half the yolk. The external part of the 

 irerminal membrane for some distance from the edue is thicker than the central 

 portion : this tliicker portion rests upon the yolk, while the central part is separated 

 from the yolk by the flat cavity already mentioned. This cavity is not exactly in the 

 centre of the membrane, the thicker external ring extending farther inwards (on the 

 right of Fig. 5) at one point than elsewhere : this broadest part of the germinal ring is 

 also the thickest part, and it forms the rudiment which will give rise to the upper or 

 dorsal })art of ilie body of the young lish. It nni.st Ije observed tliat tlie layer of yolk 

 segments extends jyari passu witli the germinal inenibrane, aud that llic patches ni oil 

 'dobules are alwavs at the edjjfe of the extendinsf membrajie. 



As the germinal membrane continues to extend the circumference of the germinal 

 ring of course gets smaller ; the rudiment of the young fish above menticjned remains 

 with its internal extremity in the same position, continually increasing in length as the 

 edge of the nuMul)rane extends over the yolk, until when the yolk is completely 

 enclosed, the whole of the germinal ring has been taken uj) into this ludiment. The 

 dorsal rudiment by this time has become much thicker and forms an almost cylindrical 

 rod, from the sides of which the germiiuil membrane extends over the yolk. The oil 

 ♦^lobules are now arranged beneath the sides of the dorsal rudiment, while the volk 



