THE RECAPITULATION THEORY 315 



this suggestion will not commend itself to embry- 

 ologists. The distinction between the yolk granules 

 and the cells in which they are embedded is a real 

 and fundamental one ; but I see no reason for 

 regarding the yolk cells as other than originally 

 functional endoderm cells in which yolk granules 

 have accumulated to such an extent that they have 

 in extreme cases become devoted solely to the 

 storing of food for the embryo.* 



Of all the causes tending to modify development, 

 tending to obscure or falsify the ancestral record, 

 food yolk is the r most frequent and the most im- 

 portant ; its position in the egg determines the 

 mode of segmentation, and its relative abundance 

 affects profoundly Jhe entire embryonic history, and 

 decides at what particular stage, and of what size 

 and form, the embryo, shall hatch. 



The loss of food_yolk is another disturbing 

 element, the full influence of which is as yet im- 

 perfectly understopd,J)ut the possibility of which 

 must be always kept in mind. It is best known 

 in the case of mammals, where it has led to appa- 

 rent, though very, ^deceptive, simplification of 

 development ; jmd it will probably not be until the 

 embryology of the large-yolked monotremes is at 

 length described, that we shall fully understand the 

 formation of the germinal layers in the higher 

 placental mammals. Amongst invertebrates we 

 know but little as yet concerning the effects of loss 

 of food yolk. It has been suggested that the 



* Cf. E. B. Wilson, " The Development of Renilla," Phil 

 Trans. 1883, p. 755. 



