362 REPRODUCTION 



with the pharynx and mouth and opens below upon the ex- 

 ternal body surface (anus). The liver and pancreas are de- 

 veloped from protrusions from the sides of the duodenum. 



The bladder has been seen to be that part of the allantois 

 which is constricted off and remains in the body. 



The lungs are developed from the esophagus and at first 

 lie in the abdominal cavity; but the formation of the dia- 

 phragm fixes them in the thorax. 



The kidneys are developed from the Wolffian bodies. 

 These bodies are 'embryonic structures only. Each is a tube 

 lying parallel to the vertebral column on either side of it. 

 This tube consists of a collection of tubules, which unite to 

 form a common excretory duct. This duct joins the corres- 

 ponding one from the opposite side to empty into the alimen- 

 tary canal opposite the allontoic stalk. Outside the Wolffian 

 bodies are two other ducts, the ducts of Miiller. They also 

 enter the intestine. 



The WolfBan body finally gives place to the kidney, from 

 which the ureter is developed. 



In the female the ducts of Miiller become the tube, uterus 

 and vagina. In the male they atrophy. 



Just behind the Wolffian bodies are developed the ovaries 

 or the testes, as the case may be. 



The development of a few of the organs has thus been 

 simply referred to. 



Satisfactory explanation of these procedures can be given 

 only in extended works on embryology, and this section may 

 be closed with the subjoined table of development, which is 

 abbreviated from one by Heisler: 



First Week. Segmentation and passage of ovum to 

 uterus. 



Second Week. Ovum in uterus. Decidua reflexa present. 

 Entoderm and ectoderm layers formed also mesoderm. 

 Embryonal area, primitive streak in primitive groove. Cho- 



