INTRODUCTION 11 



or less yolk. In the case of mammals (excepting the monotre- 

 mata: Ornithorhynchus, Echidna, etc., 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 telolecithal. 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- 



