oG FORMATION OF THE LAYERS. 



the part of the egg usually considered as food-yolk, a num- 

 ber of bodies, which eventually developed a nucleus and be- 

 came cells, and that these cells entered into the blastoderm. 

 These observations demonstrate that in the eggs of Loligo the 

 so-called food-yolk is merely equivalent to a part of the egg 

 which in other cases undergoes segmentation. 



The observations of Dr Gotte have a similar bearing. He 

 made out that in the eggs of the Hen no sharp line is to be 

 found separating the germinal disc from the yolk, and that, 

 independently of the normal segmentation, a number of cells 

 are derived from that part of the egg hitherto regarded as 

 exclusively food-yolk. This view of the nature of the food-yolk 

 was also advanced in my preliminary account of the develop- 

 ment of Elasmobranchs\ and it is now my intention to put 

 forward the positive evidence in favour of this view, which is 

 supplied from a knowledge of the phenomena of the develop- 

 ment of the Elasmobranch ovum; and then to discuss how far 

 the facts of the growth of the blastoderm in Elasmobranchs 

 accord with the view that their large food-yolk is exactly 

 equivalent to part of the ovum, which in Amphibians undergoes 

 segmentation, rather than some fresh addition, which has no 

 equivalent in the Amphibian or other holoblastic ovum. 



Taking for granted that the ripe ovum is a single cell, the 

 question arises whether in the case of meroblastic ova the cell 

 is not constituted of tvvo parts completely separated from one 

 another. 



Is the meroblastic ovum, before or after impregnation, com- 

 posed of a germinal disc in which all the protoplasm of the cell 

 is aggregated, and of a food-yolk in which 7io protoplasm is 

 present? or is the protoplasm present throughout, being simply 

 more concentrated at the germinal pole than elsewhere? If the 

 former alternative is accepted, we must suppose that the mass of 

 food-yolk is a something added which is not present in holoblas- 

 tic ova. If the latter alternative is accepted, it may then be 

 maintained that holoblastic and meroblastic ova are constituted 

 in the same way and differ only in the proportions of their con- 

 stituents. 



^ Quart. Journ. of Micr. Science, Oct. 1874. 



