490 EMBEYOLOGY OF THE LOWEE VEETEBEATES ch. 



which the eggs are rich in yolk it is readily seen that there are 

 conspicuous differences between different parts of the embryo's 

 tissues in regard to the yolk contained in their cells, for example 

 endodermal structures are frequently marked out by larger yolk 

 granules which cause them to stain more deeply with yolk-staining 

 dyes. The condition of the yolk in a tissue may indeed give a 

 useful hint as to the cell layer to which it belongs and as a matter of 

 fact evidence of this kind has played a conspicuous part in many 

 embryological discussions. 



It is important to bear in mind however the physiological 

 significance of the character in question. It appears to be closely 

 related to the metabolic processes in the tissue concerned. As a 

 given tissue in a yolky embryo goes on with its growth and 

 development its yolk is gradually used up, a necessary preliminary 

 being its breaking down into fine particles easily assimilable. 

 Tissues or cells undergoing active growth and multiplication have 

 their yolk in this fine-grained condition : those which are for the 

 time being comparatively inert retain their yolk in a coarse-grained 

 form. . Thus a disturbing factor is introduced which has to be care- 

 fully borne in mind when using the character of the yolk as a 

 criterion of the morphological nature of a' given cell or tissue. 



A still further disturbing factor lies in the fact that while yolk 

 is being used up and disappearing from view in one part of the body 

 it may be deposited in cells elsewhere — as for example takes place in 

 eggs during their period of growth within the ovary. Such increase 

 in the amount of yolk however, accompanied commonly by increase 

 in the size of the individual granules, is naturally relatively rare 

 in comparison with the breaking down of yolk which is occurring 

 through the general tissues of the embryo. , It follows that on the 

 whole coarsely granular yolk in a cell or tissue affords more reliable 

 evidence as to its nature than does fine-grained yolk — which may be 

 and usually is merely a symptom of active metabolism. 



(4) Ebcapitulation. — The fascination as well as the philosophical 

 interest of the study of Vertebrate embryology rests in great part upon 

 the recapitulation of phylogenetic evolution during the development of 

 the individual. In the early days of evolutionary embryology this 

 idea was accepted in an unquestioning and uncritical spirit and it 

 was supposed that all that had to be done to obtain an accurate and 

 fairly complete picture of the phylogenetic history of any particular 

 animal was simply to work out its ontogenetic development. The 

 more extensive knowledge which we have regarding embryological 

 phenomena to-day serves on the one hand to confirm fully the truth 

 of the general principle and on the other hand to indicate how its 

 working is interfered with by various disturbing factors. 



The main controlling factor in ontogeny is the character of the 

 adult. This is the motive power throughout the developmental period. 

 Just as according to Newton's First Law a moving body tends to 

 continue in a state of uniform motion in a straight line, so in 



