INTRODUCTION 23 
The little embryo then has an inside layer of cells as well as an 
outside layer, and is called a gastrula. The cells of the pressed-in 
part are destined to give rise to the stomach, intestine and diges- 
tive glands. On the other hand the outer surface of the embryo 
gives rise to the skin and nervous system. 
All animals from the sponges up to man pass through a gas- 
trula stage in which the body consists only of an outer and an inner 
layer. In many cases, however, as in worms, crustaceans, insects, 
mollusks, amphibia, reptiles and birds, the egg contains more or 
less yolk to provide nutriment for the developing embryo, and this 
introduces modifications of the process of cleavage, and formation 
of the gastrula so that it has taken long and careful study to 
discover that all embryos are at one time in the gastrula stage. 
Indeed, Huxley discovered that the jellyfishes were practically gas- 
trula even when adult, for they have only a digestive cavity and an 
outer skin. In all forms above the jellyfishes, however, we find a 
system of cells which come to lie between the digestive sac and the 
outer skin, and which gives rise to the muscles, and in higher forms 
to the skeleton. Although it will be impossible to do more than to 
present this crude sketch of the general processes of embryology, 
one will find a most fascinating field for thought in the study of 
such admirable works as ‘The Cell in Development and Inher- 
itance,” by Edmund B. Wilson, or the “ 'Text-Book of the Embry- 
ology of Invertebrates,” by Korschelt and Heider, Macmillan Co. 
A most readable and yet condensed discussion of general em- 
bryology is given by Richard Hertwig in “Manual of Zoology,” 
1902, translated by J. S. Kingsley, p. 159-164. 
A list of a few of the more general and popular works upon our 
marine animals may be of aid to those who desire to seriously study 
the subject 
For beginners who are engaged in collecting marine animals, 
and who desire to learn of their relationships, habits, structure and 
names, we would suggest: 
EK. C. and A. Agassiz: Seaside Studies in Natural History, Tick- 
nor and Fields, 1865. 155 pages; 186 illustrations. A clear, 
popular, account of the structure and habits of our jellyfishes, sea 
anemones and star fishes and sea-urchins. 
C, B. and G. C. Davenport: Introduction to Zoology, Macmil- 
