THE PRINCIPLES OF 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 
blastula, 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 O. 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, 7.2. 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 Paleontology, 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 paleos- 
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 organs of the vertebrate 
body. In this respect, then, embryology has proved of the greatest value in 
confirming the evidence of relationship between the paleostracan and the 
vertebrate, given 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 stage of the embryo three layers are formed, the epiblast, mesoblast, and 
hypoblast; that in all animals above the Protozoa these three layers are 
