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 onter surface of the embryo 

 gives rise to the skin and nervous S3'stein. 



All animals from the sponges up to man pass through a gas- 

 trnla 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 3'olk 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- 

 trulce even when adult, for they have only a digestive cavity and an 

 outer skin. In all forms above the jellyfishes, howev^er, we find a 

 system of cells which come to lie between the digestive sac and the 

 outer skin, and wliich 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. 139-164. 



A list of a few of the more general and popular Avorks 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 Avould suggest : 



E. C. and A. Agassiz : Seaside Studies in Natural History, Tick- 

 nor and Fields, 1805. 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- 



