An interesting chapter of vertebrate embryology deals with the environmental conditions amid which the various types of vertebrate pass through their early stages and the ways in which the young vertebrate is adapted to these conditions. In certain cases the environment and the young individual's relations to it present no special peculiarities. The young crossopterygian or lungfish or urodele leads a normal kind of aquatic existence and the strikingly uniform type of larva in these three relatively archaic vertebrates suggests strongly that it repeats an early stage of vertebrate evolution. In most verte brates, however, development is either embryonic or secondarily larval. In elasmobranchs. in teleosts and in reptiles and birds, the early stages are passed within the shelter of egg-envelopes and this involves the modifications associated with the storing up of a supply of yolk—modifications which still persist in cases such as the majority of teleosts, where a larval mode of development is re-acquired. Embryonic development is seen in its highest expres sion in the terrestrial vertebrates and these, in addition to the immense exaggeration of the ventral part of the endoderm to store up yolk (yolk-sac), show two other striking peculiarities: (I) the body of the embryo becomes enclosed in a water-jacket (amnion) in which it floats suspended and is thus protected from the sudden jars incidental to a terrestrial existence, and (2) the allantois—the pouch-like outgrowth from the hinder part of the alimentary canal which in the amphibian functions as a urinary bladder—becomes precociously enlarged and, spreading round the inner surface of the egg-shell, constitutes the breathing organ during a large part of embryonic life. The highest degree of adaptation to the terres
trial existence is reached by the ordinary mammals, in which the egg, instead of being laid at an early stage of development, is retained within the mother's uterus for a prolonged period during which the embryo passes through all the earlier helpless stages of its development. The yolk-sac, amnion and allantois are still pres ent as in the reptiles. But the allantois has developed a new func tion, that of absorbing nourishment from the uterine wall, and in correlation with this, the supply of yolk, which in the reptilian egg was so conspicuous, has now disappeared practically entirely. The yolk-sac still retains its old features but it now contains merely lymph. And, correlated with this in turn, the mammalian macrogamete or zygote has shrunk to a size comparable with that of Amphioxus. Whereas the unsegmented egg of an ostrich meas ures as much as 85 mm. in diameter that of man has reverted to as little as 0.25 mm.
Graham Kerr, A Text-book of Embryology, vol.
"Vertebrata with the exception of Mammalia" (London, 1919) ; this book gives an account of the general principles of Vertebrate Embry ology based on the study of the lower vertebrates and includes the results of modern investigations upon the embryology of some of the more primitive types. Includes a guide to the bibliography of the sub ject and to its practical technique ; A. Brachet, Traite d'Embryologie des Vertibres (Paris, 1921), a good modern text-book on Embryology of Vertebrates, including the Mammalia; contains bibliography.
G. K.)