210 MR GEORGE BROOK ON THE 



yellowish tint. The appearance in the living egg is thus explained. The 

 transparent portions between the yolk spheres are not vacuoles, and a system 

 of tubes pushed down into the yolk, but the channels by which the germinal 

 protoplasm (of which Kupffer had no cognisance) makes its way to the 

 surface. An hour after fertilisation (at 41° F.) a considerable quantity of the 

 germinal protoplasm has collected at the surface of the yolk, and forms a 

 distinct layer, varying from '0188 to "0564 mm. in thickness. At first this is 

 quite clear and homogeneous, but soon fine granules make their appearance, 

 and the whole layer becomes darker in tone. 



From the moment that a layer of protoplasm has collected at the surface, 

 an interesting series of phenomena is commenced, which is only terminated 

 when the whole of the nutritive yolk has been consumed. The germinal pro- 

 toplasm begins to grow at the expense of the yolk. Large masses of yolk are 

 incorporated within the substance of the protoplasm and digested there. 

 During this time the protoplasm is in constant motion, and flows slowly in 

 thicker and thinner waves around the yolk. These phenomena are not new, but 

 form part of a process which has frequently been described in connection with 

 the parablast. They are, however, more marked and easily followed in the 

 herring than in any other form with which I am acquainted. The process is 

 one of intracellular digestion, and at a later stage probably forms an important 

 mode in which the food yolk is used up in most meroblastic ova. The im- 

 portant point to be noted for the present is, that in the herring the yolk is 

 partly consumed to form the germinal disc itself. Kupffer supposed that 

 nearly the whole of the germinal disc was formed in this manner, but it is 

 probable that he had not studied sections of the egg in the earliest stages of 

 development. He says, to begin with, that the formative yolk appears as a 

 continuous superficial layer. It has already been seen that this is not so. 

 When the greater portion of the germinal protoplasm has collected at the 

 surface of the yolk, the appearance in an optical section of the living egg is no 

 doubt as Kupffer describes. Optical sections are, however, very misleading, 

 and should only be used in helping to explain the appearance shown in actual 

 section. A little experience may teach one how to interpret optical sections, 

 but this experience can only be obtained from a study of actual sections of 

 the egg. 



Fig. 6 represents an optical section of the living egg of the herring an 

 hour after fecundation. The germinal protoplasm is seen as a continuous 

 superficial layer, which is considerably thicker at one side. The protoplasm is 

 filled with fine and larger granules, and a number of small clear vesicles may 

 also be made out under a moderately high power. The superficial layer of 

 "yolk granules " has entirely disappeared, and the yolk now consists of a mass 

 of large yolk spheres, which appear to be evenly distributed throughout. A 



