14 SUMMARY OF CURRENT RESEARCHES RELATING TO 



plasm, and nuclear spot ; others havo a dark margin, a fat-liko 

 cortex, and paler contents. Notwithstanding these differences thero are 

 some indications of the passage of tho former into the latter state. The 

 germinal spots arise from the nodal points of the nuclear framework ; 

 when they multiply, the larger germinal spot produces a brood by 

 gemmation and fission ; differences are exhibited in different groups of 

 animals. In consequence of their amoeboid nature, germinal spots 

 which have becomo independent are capable of uniting into columns, and 

 it must, therefore, be supposed that the transversely striated cords are 

 not always directly due to the multiplication of germinal spots. 



The membrane of the germinal vesiclo may present differences in 

 one and the same animal ; for example, in Triton it may be proportion- 

 ately thick and perforated, or it may be thin and apparently without 

 pores, and possibly it may disappear altogether. 



The name of mantle-layer is applied to a layer of germinal vesicles, 

 first described by Einier in reptiles ; it is only temporarily present, and 

 presents numerous variations ; it consists of granules, which look like 

 germinal spots, and are often so grouped as to seem to have a radial 

 striation. An account is promised of observations which seem to show 

 that this layer is connected with processes of germinal spots. Among 

 the general structural relations of the egg we must reckon the cavity 

 around the germinal vesicle, which is filled by a clear, very soft and 

 almost fluid protoplasm ; from this space hollow passages extend into the 

 yolk, where they vary considerably in form and direction. This cavity 

 was first noticed by Pfluger. 



The germinal vesicle, which is ordinarily spherical, may be seen in 

 the fresh state to exhibit depressions and processes, or pits and lobes, 

 which may be regarded as due to movements. But it must remain 

 uncertain whether this change in form is due to the vesicle itself or to 

 the whole egg-cell. 



The yolk consists of spongioplasm and homogeneous hyaloplasm, to 

 which are added vitelline granules and spheres. The spongioplasm is 

 generally a fine closely-felted network, without any regular arrangement, 

 but in others there are pretty regular concentric lines, or radially 

 arranged bands. The intermediate spaces vary in size, but are often 

 very small ; in addition to these there may be larger cavities arising 

 from the germinal vesicle, extending through the yolk in a radial 

 manner, and anastomosing with one another. It is erroneous to suppose 

 that the spaces seen by Reichert in the yolk of bony fishes were due to 

 coagula. When larger yolk-spheres appear and become regularly 

 arranged in the periphery of the egg we can distinguish an outer from 

 an inner yolk. It has often been supposed that nuclear and cellular 

 structures may be seen in the yolk, before the commencement of segmen- 

 tation. These bodies are of two kinds ; some resemble germinal spots, 

 while the others are like thickenings of the nodal points of the spongio- 

 plasm. The former are really germinal spots, which have passed into 

 the yolk ; the others resemble the secondary nuclei of other cells, and of 

 the egg of Ascaris megalocephala. As to what becomes of them there is 

 some difficulty, but it seems to be certain that they do not form the 

 material for the membrana granulosa. Prof. Leydig's own observations, 

 supported by those of Heider and Blochmann on Arthropod ova, lead 

 him to suppose that they form a cellular layer round the yolk, but that 

 the boundaries between the cells are not well marked : the " internal 



