THE PRINCIPLES OE EMBRYOLOGY 485 



In cases where there is no yolk, or very little, as in Lucifer and 

 Amphioxus respectively, the embryo is compelled to feed itself at a 

 very early age ; such embryos form a free-swimming pelagic ciliated 

 bias tula, the invagination of which, for the purpose of collecting food 

 material out of the open sea, is the simplest method of obtaining 

 nutriment. Here, as in other cases, it is the physiological necessity 

 which determines the method of formation of the gut, and such 

 similarity of appearance as exists between the gastrula of Lucifer and 

 that of Amphioxus, by no means implies that the gut of the adult 

 Lucifer is homologous with the gut of Amphioxus. 



I have compared two meroblastic eggs of the two classes respec- 

 tively, because the scorpion's egg is meroblastic. I imagine that no 

 real difficulty arises with respect to holoblastic eggs, for the experi- 

 ments of 0. Hertwig and Samassa show that by centrifugalizing, 

 stimulating, and breaking down of large spheres the holoblastic 

 amphibian egg may be converted into a meroblastic one, and then 

 development will proceed regularly, i.e. in this case also the growth 

 proceeds from the animal pole ; the large cells of the vegetal pole, like 

 the yolk-cells of the meroblastic egg, manufacture pabulum for the 

 growing syncytial host. 



Summary. 



Any attempt to discover how vertebrates arose from invertebrates must be 

 based upon the study of Comparative Anatomy, of Palaeontology, and of Embryo- 

 logy. The arguments and evidence put forward in the preceding* chapters 

 show most clearly how the theory of the origin of vertebrates from paheos- 

 tracans is supported by the geological evidence, by the anatomical evidence, 

 and by the embryological evidence. Of the three the latter is the strongest 

 and most conclusive, if it be taken to include the evidence given by the larval 

 stage of the lamprey. 



The stronghold of embryology for questions of this sort is the Law of 

 Recapitulation, which asserts that the history of the race is recapitulated to 

 a greater or less extent in the development of the individual. In the previous 

 chapters such recapitulation has been shown for all the org-ans of the vertebrate 

 body. In this respect, then, embryology has proved of the g'reatest value in 

 continuing - the evidence of relationship between the palfeostracan and the 

 vertebrate, g*iven by anatomical and geological study. 



There is, however, another side to embryology, which claims that the tissues 

 of all the Metazoa are built up on the same plan ; that in all cases in the very 

 early stag*e of the embryo three layers are formed, the epiblast. mesoblast, and 

 hypoblast ; that in all animals above the Protozoa these three layers are 



