316 Vertebrate Embryology 



der thread, the yolk-stalk (Figs. 103 and 104). 

 This stalk is formed from the neck of the sac, 

 which becomes greatly elongated and loses its 

 central lumen ; it enters the umbilical cord 

 near its placental end and passes through it to 

 its attachment to the intestine. The yolk-sac 

 and stalk are so small in proportion to the 

 other structures, during the latter periods of 

 pregnancy, that they are easily lost sight of. 



DEVELOPMENT OF THE EXTERNAL GENITALIA 



IN MAN 



In the frog and chick there are no structures, 

 save the cloaca, that could be called " external 

 genitalia," so that it may be well, here, to give 

 a brief summary of the development of these 

 structures in the human embryo. 



Until about the fifth week of development 

 there is in man, as in the adult frog and chick, 

 a common external opening, the cloaca, for 

 both the rectum and the urogenital organs. 

 Towards the end of the fifth week, before the 

 completion of the septum (the future perineum) 

 dividing the cloaca into an anterior portion, 

 the uro-genital sinus, and a posterior portion, 

 the anus, the anterior wall of the uro-genital 

 sinus thickens to form a blunt projection, the 



