INTRODUCTION 11 
or less yolk. In the case of mammals (excepting the monotre- 
mata: Ornithorhynchus, Echidna, ete., which have large ova) 
the yolk is scanty in amount, and quite uniformly distributed 
in the form of fine granules; the ovum is, therefore, relatively 
very small (mouse, 0.059 mm.; man, 0.17 mm.). Such ova are 
often termed alecithal, which means literally without yolk. In 
the literal sense, however, no ova are entirely alecithal, so that 
it will be better to use the term of Waldeyer, isolecithal. In the 
amphibia the yolk is much greater in amount and it is centered 
towards one pole of the ovum; the germinal vesicle (nucleus of 
the egg-cell), which occupies the center of the protoplasm of the 
ovum, is therefore displaced towards the opposite pole of the 
ovum. Such ova are termed felolecithal. In the ova of Selachia, 
reptiles and birds, the yolk is very much greater in amount and 
in consequence the protoplasm containing the germinal vesicle 
appears as a small disc, the germinal disc, on the surface of the 
huge yolk-mass. 
But no matter how large the ovum may become by deposi- 
tion of yolk, its unicellular character is not altered. The deposi- 
tion of yolk is simply a provision for the nutrition of the embryo. 
In the mammals the nutrition of the embryo is provided for by 
the placenta; therefore yolk may be dispensed with. In the 
absence of such provision the amount of yolk is a measure of the 
length of the embryonic period of development. In the amphibia, 
for instance, this is relatively brief, for the yolk is soon used up, 
and the larva must then depend on its own activities for its nutri- 
tion. Therefore the development involves a metamorphosis: the 
embryo is born in a very unfinished condition, as a larva (the 
tadpole in the case of amphibia), which must undergo an exten- 
sive metamorphosis to reach the adult condition. In the reptiles 
and birds, however, the amount of yolk is sufficient to carry the 
development through to a juvenile condition, before an extrane- 
ous food-supply is necessary. The metamorphosis, therefore, 
which takes place in free life in amphibia, goes on within the egg 
in reptiles and birds. The first form of development is known 
as larval, the second as foetal. 
The amount and arrangement of yolk also influences very 
profoundly the form of the early stages of development. Ova 
are Classified in this respect as holoblastic and meroblastic. Holo- 
blastic ova are those in which the process of cell division (cleav- 
