ix YOLK 489 



On the recapitulation hypothesis the segmentation and other 

 early stages of ontogeny represent ancestral evolutionary stages 

 common to all Vertebrates. The differences to be observed between 

 such stages in different members of the group are consequently not 

 to be looked on as ancestral but rather as due to the influence of 

 disturbing secondary factors. Of these by far the most important is 

 the presence of the particles of yolk, this dead substance clogging 

 and retarding the living activity of the egg protoplasm. The extent 

 to which it does this in any particular region of the egg is roughly 

 proportional to its relative amount as compared with the living 

 protoplasm in that portion of the egg. The yolk is as a rule of 

 higher specific gravity than the protoplasm. Correlated with this it 

 tends to be in proportionally greater amount in the lower parts of 

 the egg than in the apical part, with the result that the processes of 

 cell division and of development generally are relatively more slowed 

 down in these lower portions — in extreme cases brought to a full 

 stop — by its retarding influence. Typical examples of this are seen 

 in the holoblastic but unequally segmenting eggs of the ordinary 

 Amphibia. In this case it is possible by replacing experimentally 

 the action of gravity by a more potent force (by centrifugalizing the 

 eggs) to concentrate the yolk still more than is natural in the lower 

 hemisphere with the result that the egg is now converted into a 

 meroblastic one (0. Hertwig) the lower hemisphere being unable to 

 segment. On the other hand by inverting .the egg and so allowing 

 the yolk granules to settle down towards the apical pole under the 

 influence of gravity it is possible to cause the segmentation furrows 

 to start from the abapical pole and spread towards the apical. 



The influence of yolk upon the gastrulation process will have 

 been realized from the perusal of Chapter II. : it is well illus- 

 trated by the series Amphioxus (Fig. 18), Petromyzon (Fig. 23), 

 Bana (Fig. 25), Zepidosiren (Fig. 21), Hypogeophis (Fig. 27) and 

 Torpedo (Fig. 28). Put in a single sentence it may be said to 

 consist above all in the gradual subordination of the process of 

 invagination to those of overgrowth and delamination. In the 

 succeeding stage it makes itself apparent more particularly in the 

 modification of the mode of origin of the mesoderm, the outgrowth 

 of hollow enterocoelic pouches being replaced by the delamination of 

 a solid mass or sheet. 



The storage of yolk carries with it not merely the modifications 

 just indicated in the processes of segmentation and gastrulation. Its 

 influence becomes retrospective and affects even preceding stages 

 during the growth of the intra-ovarian egg. This is shown more 

 especially by the precocious concentration of yolk in that portion of 

 the egg which will later become endodermal. Thus is the telolecithal 

 condition brought about and telolecithality itself is seen to be really 

 a foreshadowing of a particular adaptive feature of later stages of 

 development (p. 183). 



In examining sections of later stages of Vertebrate embryos in 



