xiv ELEMENTS OF VERTEBRATE EMBRYOLOGY 455 



of the blastula, which in Amphioxus becomes invaginated to form the 

 "endoderm, is enormously increased in thickness and it is physically 

 impossible for it to be inva-inated within the relatively mini: 

 celled portion of tin Macula. Consequently we find with increasing 

 proportion of yolk in the abapical region of the egg that the process of 

 invagination becomes less and less conspicuous, its place being 

 a greater and greater activity in the process of overgrowth by the head- 

 ward portion of the gastrular rim or lip. In the more heavily yolkl 

 eggs still another process becomes apparent namely delami nation, the 

 small-celled portion of the egg's surface gradually extending by tin- 

 addition to it of small cells split off from the large yolky cells of the 

 abapical region. The result of these various processes acting together 

 is that the egg with unequal segmentation eventually has its large-celled 

 portion (endoderm) completely enclosed within a covering of small cells 

 (ectoderm) just as is the case in the completed gastrula oiAmphioxus. 



In the various groups of lower vertebrates we find illustrated in 

 varying degrees these modifications of the processes of segmentation and 

 gastrulation brought about by the presence of yolk. In Amphioxus as 

 already indicated these modifications are seen at their minimum. A 

 series such as Polypterus, a Lamprey, a Newt (Triton) or Frog (Rana), a 

 Lepidosiren, shows increasing degrees of this modification. In the large 

 egg of the Gymnophiona the segmentation of the abapical part of the 

 egg is delayed to such an extent that the early stages of segmentation 

 give the impression of a meroblastic egg. In the Elasmobranchs the egg 

 is still larger and richer in yolk and we find a typical meroblastic condi- 

 tion. In the Teleosts, in correlation probably with the great wastage 

 of their eggs, the eggs have become greatly increased in number and 

 correspondingly decreased in size and therefore in yolk-content. How- 

 ever, the proportion of yolk to living cytoplasm is unaffected and con- 

 sequently the teleostean egg retains the typical meroblastic character. 



The special advantage of a store of yolk in the egg is that it enables 

 the young individual to live for a more or less prolonged period upon 

 its capital, so to say, and thus postpone the period when it has to enter 

 into the active struggle for food and for existence. It is able to pass 

 through in seclusion and comparative safety that period of its life that 

 would otherwise be passed as a larva fending for itself. In such a case 

 as that of Lepidosiren (p. 389) the larval stages are passed through in con- 

 ditions which do not depart markedly from the normal and in which all 

 the living activities of the young animal are carried on in normal fashion 

 except that it is free from the necessity of obtaining food. In such a 

 case as that of Scyllium or a Skate (Raja) on the other hand the seclusion 



