488 EMBRYOLOGY OF THE LOWEE VERTEBRATES ch. 



continuity of protoplasm is much more widely spread than is 

 generally recognized. 



(3) Yolk. — Theoretically the most primitive type of zygote 

 should from the beginning be able to absorb food for itself. As an 

 actual fact however the zygote is provisioned for a shorter or longer 

 period by the highly nutritious fat and proteid, in the form of yolk 

 which is stored up in its cytoplasm. 1 With increasing specialization 

 the amount of this store becomes greater and greater so as to 

 lengthen the period during which the young individual is provisioned 

 and freed from the necessity of working for its own living. A good 

 example of a high degree of such specialization amongst Vertebrates 

 is afforded by the relatively huge egg of the Ostrich. 



It has of course to be borne in mind that the degree of specializa- 

 tion in this direction is to be estimated not merely by the absolute 

 amount of yolk present but still more by the relative amount of yolk 

 in proportion to protoplasm. Thus two eggs may be described as 

 equally richly yolked although very different in size provided that the 

 proportion of yolk to protoplasm is similar in the two cases. In 

 correlation with this we find that a group characterized by heavily 

 yolked eggs may evolve in the direction of producing more and more 

 numerous, and therefore necessarily smaller, eggs. Good examples 

 of this are seen in the Teleostean fishes where the eggs may be 

 produced in enormous numbers and of very minute size although 

 still retaining a proportionately large supply of yolk. 



In C. Rabl's discussions of his " Theory of the Mesoderm " (1889) 

 an important place is taken by repeated losses and re-acquisitions of 

 yolk during the phylogeny of the Vertebrata. Rabl arranges 

 Cyclostomes, Elasmobranchs, Ganoids, Amphibians, meroblastic 

 " Protamniota " and Mammals, in a linear series, and concludes 

 that Ganoids and Amphibians have undergone a diminution of yolk 

 and have therefore reverted to the holoblastic condition ; that mero- 

 blastic Protamniota have re-acquired a large amount of yolk and 

 have therefore reverted to the meroblastic condition ; arid that 

 finally Mammals have lost their yolk and again become holoblastic. 

 In the opinion of the writer there is no sufficient justification for 

 any one of these assumptions except the last. There is, as is well 

 known, definite evidence to show that Mammals are descended from 

 ancestors with large and heavily yolked eggs and that the small size 

 and practically yolkless character of their holoblastic eggs are 

 secondary acquirements. In this case the loss of yolk has brought 

 in its train profound changes in the early processes of development 

 but of such there are no signs in those other cases in which Eabl 

 supposes loss of yolk to have taken place. It must also be remembered 

 that in the Mammal there is an obvious physiological reason for 

 the loss of yolk — namely that the food material needed during the 

 development of the embryo is provided from the tissues of the mother. 



1 For a detailed account of the development of the yolk in the egg of one of the 

 lower Vertebrates {Proteus) see Jorgensen (1910). 



