414 OUTLINES OF CHORDATE DEVELOPMENT 



The chin gradually enlarges, and the lips, now both complete 

 medially, continue to enlarge (seventh week). At about eight 

 weeks (Fig. 172, B) the eyelids are forming, and the eyes, now 

 rapidly approaching one another, are separated from the fore- 

 head by oblique supraorbital folds. The ears now are marked 

 by well-developed pinnae, but still lie far down toward the neck, 

 below the level of the mouth. The mouth is less extended 

 transversely and the nose is completely separated from the 

 forehead. 



During the next week or ten days (Fig. 172, C), the eyelids 

 close, the eyes move closer together, and the height of the fore- 

 head increases. The nose, though still very broad, begins to 

 project slightly, and the external nares become temporarily 

 closed by epidermal proliferations. The ear gains a somewhat 

 higher position. The mouth is smaller, the lips thinner, and 

 the lower jaw quite prominent. During the third month (Fig. 

 172, D), the pinna reaches nearly its adult position, the nose 

 projects markedly, the lips, especially the upper, become thin- 

 ner and protruded, and the essentials of the adult physiognomy 

 are fairly established. 



THE DEVELOPMENT OF THE EXTERNAL GENITALIA 



The end of the gut posterior to the origin of the allantois 

 (see below) forms the dilated cloaca, which is separated from 

 the surface of the body by a thin portion of the body wall 

 known as the cloacal membrane (Fig. 173, A). This membrane 

 is later depressed below the surface of the body, at the bottom 

 of a shallow depression (proctodseum). The cloacal cavity 

 becomes divided into a ventral portion, the urinogenital sinus, 

 receiving the openings of the excretory and reproductive ducts 

 and the allantois, and the rectal portion. The cloacal mem- 

 brane is correspondingly divided into the urinogenital mem- 

 brane and the anal membrane, the two being separated by a 

 narrow bridge of tissue forming the perineal rudiment. 



In order to find the earliest traces of the external genitalia, 

 we must go back to the embryo of the early part of the second 



