FORMATION OF THE GERMINAL LAYERS IN TELEOSTBI. 219 



Having concluded that the third furrow takes an equatorial direction in 

 the herring ovum, it will be well to reflect on the significance of this fact. The 

 main portion of the germinal protoplasm, which constitutes the archiblast, 

 forms the animal pole of the egg, while the yolk, together with the residual 

 protoplasm, is to be regarded as the vegetative pole. The animal pole at this 

 stage consists of four segments or cells, while the whole of the vegetative pole 

 has the value of one cell. The whole of the vegetative area may be compared to 

 a gigantic fat cell in which the fat is replaced by food yolk. The function of 

 the cortical protoplasm is to digest and absorb the food material as fresh 

 nourishment is required by the growing organism. Having once grasped the 

 significance of this point, the interpretation of future developmental phases does 

 not present much difficulty. 



The ovum of an amphibian is holoblastic, while that of the herring is mero- 

 blastic, yet this difference in the mechanical division of the ovum does not pre- 

 vent a comparison of the two types. In an amphibian ovum, such as that of 

 Rana, the majority of the germinal protoplasm has collected in the animal pole 

 by the time that the first equatorial furrow is formed. The same is the case in 

 the herring ovum. In Rana the vegetative pole consists at first of four large 

 segments, which contain the greater portion of the yolk material, but which are 

 also supplied with a considerable amount of protoplasm. The fact that at this 

 stage the vegetative area consists of four segments instead of one, shows that 

 the proportion of yolk to protoplasm is not so great as to entirely prevent the 

 segmentation process from progressing. The proportion, indeed, is such that, 

 in accordance with the law of segmentation, the division of the vegetative area 

 is slower, and the resulting segments are larger, than is the case in the animal 

 pole. It is also important to note, whilst making this comparison, that the 

 protoplasm in the vegetative area of the amphibian ovum is not collected in any 

 particular part, but that it is generally distributed throughout each segment. 

 The same is the case with the yolk material. With further subdivision, there- 

 fore, each cell in the yolk pole consists partly of yolk and partly of proto- 

 plasm. 



Thus each cell carries its food supply along with it. Such is not the case 

 in the herring ovum, and it is this fact which constitutes the essential difference 

 between the two types. The food yolk in the herring ovum does not segment. 

 This absence of segmentation in the yolk of fishes arises from two causes — 

 viz., the overwhelming preponderance of food yolk, and the absence of a suffi- 

 cient quantity of protoplasm distributed through it. In the herring ovum the 

 protoplasm in the yolk pole consists of a comparatively thin cortical layer, with 

 a few branching processes pressing into the food material, which are almost 

 entirely confined to that portion of the vegetative area on which the archiblast 

 rests. From the nature of this distribution, the food supply cannot be assimi- 



