FORMATION OF THE LAYERS. 269 



As soon as the fact is recognised that both meroblastic and 

 holoblastic eggs have the same fundamental constitution, the 

 admission follows, naturally, though not necessarily, that the 

 eggs belonging to these two classes differ solely in degree, not 

 only as regards their constitution, but also as regards the manner 

 in which they become respectively converted into the embryo. 

 As might have been anticipated, this view has gained a wide 

 acceptance. 



Amongst the observations, which have given a strong objective 

 support to this view, may be mentioned those of Professor 

 Lankester upon the development of Cephalopoda 1 , and of 

 Dr Gotte 2 upon the development of the Hen's egg. In Loligo 

 Professor Lankester shewed that there appeared, in the part 

 of the egg usually considered as food-yolk, a number of bodies, 

 which eventually developed a nucleus and became 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 3 , 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 



1 Annals and Magaz. of Natural History, Vol. xi. 1873, p. 81. 



2 Archiv f. Mikr. Anat. Vol. x. 



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



