168 EDWARD MCCRADY, JR. 



The oral shield. Figure 53, A, shows a front view of a 

 curious, cornifield epithelial structure which develops around 

 the oral aperture during the last day of gestation. This ap- 

 parently useless structure is the most conspicuous and char- 

 acteristic external feature of stage 34. Selenka (1887, S. 157) 

 first described and figured it, and called it the 'Schnabel- 

 schild.' His account is quoted below with the exception of 

 his timing data, which are incorrect. 



Fig. 54 Urinogenital reconstruction from 17173 (stage 35), showing well- 

 developed mesonephros, only the beginning of a metanephros, and no mulleriau 

 duct. 



The most remarkable structure of the epidermis is the oral 

 shield, which is illustrated in Plate XXVII .... Some 

 hours before birth it shows signs of degeneration, and in the 

 new-born there is only a trace of it left. The complete shield 

 (fig. 5) surrounds the mouth like a flat collar. Its circum- 

 ference is raised into six prominent points, the lower ones of 

 which are quite sharp. The whole structure is composed of 

 nothing but cornified, epidermal cells. 



When I first observed this remarkable lobate structure I 

 was reminded of the horny bill of the monotreme, Ornitho- 

 rhyncus, and I believe I make no mistake in recognizing in this 



