18 Dynamic Theory. 



the future coelom or body cavity. The body is now substantially two 

 tubes, one outside, composed of the skin stratum and flesh stratum 

 with their appurtenances, and inside of this, the other and smaller one 

 composed of the vascular and mucous stratum. The body cavity, the wall 

 of which is the flesh stratum, is the holder of this intestinal tube with 

 all its subsequent derivations of heart and blood vessels, liver, lungs, 

 kidneys, &c. 



The fluid contained in the yelk sac is nutritive protoplasm called the 

 yelk, and is being used by the developing embryo to suppty the materials 

 for all the changes and growths above enumerated as well as those that 

 follow. The opening or neck between the yelk sac and the cavity of the 

 body is called yelk duct and it remains wide open until it is closed in 

 upon by being surrounded by the swelling amnion sac, and until the 

 bulk of the }'elk has been absorbed by the growing embryo. 



FIG. 23. 



FIG. 24. 



FIG. 23. Human embryo, third week. C. Chorion with tufts. E.-Embryo. Am. 

 Anmion. AL Allan tois. Y. Yelk sac. (Haeckel.) 



FIG. 24 Human embryo, fourth week. 



d. Yelk sac torn off. L Allantois. /.Arms, just budding. 



6. Legs, just budding. v. Fore-brain. z. Twixt-brain. 



m. Mid-brain. A.- Hind-brain. ?i. After-brain, 



a. Eye. k. Three gill arches. c. Heart. 



s. Tail. w. Vertebral column. (Krause.) 



The double membranes that form the yelk sac are the two that we 

 saw were formed from the original entoderm; viz. , the vascular stratum 

 and the mucous stratum. The constriction of the skin stratum causes 

 the parts of these layers that remain above the line of the constriction 

 to close up in the shape of a tube within the body cavity. This tube is 

 the rudimentary intestine and is afterward developed into the alimentary 

 canal, stomach, &c. At first it is nothing but a plain tube closed at 

 both ends and opening from its middle part into the yelk sac through 



