Mervin T. Sudler 



393 



Embryo XII. 



This embryo is 2.1 mm. long and about 13 days old; and hence it 

 represents a late two weeks' or early three weeks' embryo. The entire 

 embryo has been modeled by Dr. Mall and I have used his model and 

 the figure of it. In this embryo the pharynx is still in a very simple 

 condition. The stomodseum has not yet broken through to form the 

 mouth, the endodermal cavity shows Seesel's pocket and diverticula 



^ Bt' Br"' 



Fig. 1. Lateral view of the model of embryo XII (after Mall). Br.^ and Br.^^, 

 first and second visceral pouches; M, mouth just behind the stomodieum ; L, rudi- 

 ment of the liver; T, median thyroid rudiment. 



corresponding to the 1st and 3nd visceral pouches. The general struc- 

 tures and relations can be seen in Fig. 1. To* describe the model in 

 detail from points of view not shown in the figure : Seen from above it 

 presents a flattened slightly curved plate; the oral end resembling an 

 ordinary cloverleaf in outline. The middle leaf represents Seesel's 

 pocket, while the two lateral prominences correspond to the first pair 

 of visceral pouches. Directly behind these are two smaller lateral 

 swellings which fit into the 2nd pair of visceral pouches. These are 



