58 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 51 



Stage XX 

 Figures 23-236 (Plate XXII) 



In this surface view (fig. 23) several changes are seen, though no 

 very great advance in development has taken place. The outlines 

 of the digits (five in the manus and four in the pes) are now well 

 defined; they even project slightly beyond the general outline of the 

 paddle-shaped part. The tail has begun to straighten out, and it 

 now extends across the front of the face. The lower jaw has in- 

 creased in length, but is still shorter than the upper. The eyelids, 

 especially the upper, are beginning to be discernible in surface view. 

 Though still without pigment, the surface of the body is beginning 

 to show by faint transverse lines the development of scales; these 

 lines are most evident in this figure in the middle region of the tail, 

 just before it crosses the nose. 



A sagittal section of the entire embryo (except the tail) of this 

 age is shown in figure 23a. In the head region the section is nearly 

 median, while the posterior part of the body is cut slightly to one 

 side of the middle line. At the tip of the now well-developed snout 

 is seen one of the nostrils (an), cut through the edge; its connection 

 with the complicated nasal chamber (11) is not here seen, nor is the 

 connection of the nasal chamber with the posterior nares (pn). The 

 pharynx (ph) is anteriorly connected with the exterior through the 

 mouth (m) and the nares, while posteriorly it opens into the oesoph- 

 agus (oe) ; the trachea (ta), though distinct from the oesophagus, 

 does not yet open into the pharynx. In the lower jaw two masses 

 of cartilage are seen, one near the symphysis (mk) and one near the 

 wall of the trachea, doubtless the rudiment of the hyoid. The deep 

 groove back of the Meckel's cartilage (mk) marks the tip of the 

 developing tongue, which here forms the thick mass on the floor of 

 the mouth cavity. Dorsal to the pharynx a mass of cartilage (se) 

 is developing in the sphen-ethmoid region. This being a median 

 section, the ventricles of the fore- (fb), mid- (mb), and hindbrain 

 (hb) are seen as large cavities, while the cerebral hemispheres (ch) 

 appear nearly solid, only a small portion of a lateral ventricle show- 

 ing. The pineal gland (epi) is cut a little to one side of the middle 

 and so does not show its connection with the brain. At the base of 

 the brain the infundibulum (in) is seen as an elongated cavity whose 

 ventral wall is in close contact with a group of small, darkly staining 

 alveoli (p), the pituitary body. Extending posteriorly from the 

 pituitary body is a gradually thickening mass of cartilage (bp), 

 which surrounds the anterior end of the notochord (nt) and may be 

 called the basilar plate. In its anterior region, where the section is 



