48 OUTLINES OF EVOLUTIONARY BIOLOGY 



Coelenterata (a group which includes such forms as jelly-fish, sea 

 anemones and corals), a third layer of cells makes its appearance 

 between the first two and forms the mesoblast (giving rise to the 

 mesoderm of the adult). In Amphioxus this mesoblast arises as 

 a series of hollow, pouch-like outgrowths of the hypoblast (Fig. 13, 

 XI, XII, c.p.), and the cavities which these pouches contain give 

 rise to the body cavity or coelom, while their walls give rise to 

 ruesodermal tissues. 



In accordance with what is commonly known as the germ- 

 layer theory, these three layers of cells, epiblast, mesoblast, and 

 hypoblast, can be recognized in the embryos of all the higher 

 animals, and from them all the various parts of the adult body 

 are derived. The epiblast gives rise to the outer skin or 

 epidermis, the nervous system and the essential parts of the 

 sense-organs ; the hypoblast gives rise to the lining epithelium of 

 the alimentary canal and its various outgrowths, including the 

 digestive glands, while from the mesoblast arise the various 

 connective and skeletal tissues, the blood-vessels and the essential 

 organs of reproduction. The cells of which each tissue is com- 

 posed acquire a characteristic structure of their own, and in 

 this way the histological differentiation of the body is gradually 

 completed. 



With the early stages in the development of Amphioxus, 

 described above, we may compare the corresponding processes in 

 the life-history of a flowering plant. Here the ovum, instead of 

 being at once liberated from the parent, undergoes all the 

 earlier stages of its development within the so-called ovule, 

 which,. with the contained embryo, will ultimately be set free as 

 the ripe seed (compare Chapter VIII.). 



As a definite example we may take the common weed known as 

 shepherd's purse, Capsdla bursa-pastoris (Fig. 14). The ovum 

 lies in a cavity in the ovule termed the embryo-sac. It is at first 

 a single nucleated mass of protoplasm without any cell-wall. 

 After fertilization it divides into two cells separated by a wall, 

 and the process is repeated by a series of divisions parallel to the 

 first one until we have a row of cells. The cell at one end of 

 this row, known as the basal cell (Fig. 14, A, B, fc.c.), is larger 

 than the others and is attached to the wall of the embryo-sac. 

 At the opposite end is a rounded cell, called the embryonic cell, 

 from which the body of the young plant will be chiefly formed. 

 The remainder of the row, including the basal cell, is known as 



