212 EMBRYOLOGY. 



the structure of the same name in embryo Fishes (Plate I., fig. 7 hn), 

 for it is at this point that the dermal yolk-sac is continuous by means 

 of its stem-like elongation with the walls of the belly. As in the 

 Fishes, it surrounds an opening (Plate I., figs. 7 and 5 hn) which unites 

 the portion of the body-cavity lying within the embryo (lh l ) with 

 the extra-embryonic part lying between the embryonic membranes 

 (Mi 2 ). Furthermore, the stalk of the yolk-sac or vitelline duct, 

 which is continuous with the embryonic intestine, and which is 

 indicated in the above-mentioned figures of Plate I. by the small 

 circle dn, passes through the opening. 



The amniotic sac affords an additional special advantage to the 

 embryos of Reptiles and Birds in that an albuminous saline fluid, the 

 liquor amnii, collects in its cavity. In it the delicate, easily injured 

 embryo composed of plastic cells floats, as it were, and is able to 

 move. 



The amniotic sac is small at the beginning of its development, but 

 enlarges with each day of incubation, since it keeps pace with the 

 growth of the embryo and encloses a larger and larger amount of 

 amniotic fluid. 



At the same time its wall becomes contractile. Certain cells in its 

 somatic mesoderm develop into contractile fibres, which in the Chick 

 give rise to rhythmic movements from the fifth day of incubation 

 onward. One can observe these while the egg-shell remains intact, 

 if one holds the egg toward a source of bright light, and for this 

 purpose makes use of the ob'scope constructed by PREYER. In this 

 manner it can be determined that the amiiion. executes about ten 

 contractions in a minute, which, beginning at one pole, proceed to 

 the opposite end, like the contractions of a worm. Thus the amniotic 

 fluid is set in motion, and the embryo oscillates or rocks regularly 

 from one end to the other. The rocking of the embryo, as PREYER 

 expresses it, becomes more and more obvious in the later days 

 of incubation, since" the contractions of the amnion become more 

 energetic. 



The serosa (S) is a wholly transparent, easily ruptured membrane^ 

 which is closely applied to the vitelline membrane. It consists of two 

 thin cell-layers, which take their origin from the outer germ-layer 

 and the parietal middle layer, and like them are distinguished by 

 blue and red lines in the diagram. The serous membrane is origin- 

 ally present as a separate structure only in the region of the amnion 

 and of the embryo (Plate I., fig. 4), as far as the body-cavity is formed 

 in the middle germ-layer. It then enlarges to the same extent as the 



