EXPLANATION OF THE FIGURES. IX 



FIG. 



106-8. Sections of the embryo, showing the formation of the dorsal canal. 



109. Section, showing- the position of the embryo of a vertebrate, in relation 



to the yolk. 



110. Section, showing the same in an articulate (Podurella}. 



11^-22. Sections, showing the successive stages of development of the em- 

 bryo of the white-fish, magnified. 



123. Young white-fish just escaped from the egg, with the yolk not yet fully 



taken in. 



124, 125. Sections of the embryo of a bird, showing the formation of the 



allantois ; e, embryo ; x x, membrane rising to form the amnios ; 

 , allantois ; ?/, yolk. 



126. The same more fully developed. The allantois (a} is further developed, 



and bent upwards. The upper part of the yolk (dd) is nearly 

 separated from the yolk sphere, and is to become the intestine. The 

 heart (k) is already distinct, and connected by threads with the 

 blood-layer of the body. 



127. Section of the egg of a mammal ; t>, the thick vitelline membrane, or 



chorion ; y, yolk ; s, germinative dot ; g, germinative vesicle. 



128. The same, showing the empty space () between the vitelline sphere 



and chorion. 



129. Shows the first indications of the germ already divided in two layers, 



the serous layer (s} : and the mucous layer (m). 



130. The mucous layer (m) expands over nearly half of the yolk, and be- 



comes covered with many little fringes. 



131. The embryo (e) is seen surrounded by the amnios (), and covered by 



a large allantois (a) ; p e, fringes of the chorion ; pm, fringes of the 

 matrix. 



132. Hydra, showing its reproduction by buds. 



133. Vorticella, showing its reproduction by division. 



134. Polyps, showing the same. 



135. A chain of Salpae. 



136. An individual salpa ; m, the mouth ; a, embryos. 



137. Cercaria, or early form of the Distoma. 



138. Distoma, with its two suckers. 



139. Nurse of the Cercaria. 



140. The same, magnified, showing the included young. 



141. Grand nurses of the Cercaria, enclosing the young nurses. 



142. Stages of development of a jelly-fish (Medusa); a, the embryo in Us 



first stage, much magnified ; , summit, showing the mouth ; c, /, 

 g, tentacles shooting forth ; e, embryo adhering, and forming a pedi- 

 cle ; h, t, separation into segments ; d, a segment become free ; 

 , form of the adult. 



143. Portion of a plant-like polyp ( Campanula-rick) ; a, the cup which bears 



tentacles ; b, the female cup, containing eggs ; c, the cups in which 

 the young are nursed, and from which they issue 

 B 



