1901.] NATURAL SCIENCES OF PHILADELPHIA. 22? 



In specimens of the present stage the Anlagen of thft various 

 structures have so far differentiated that they are in most cases 

 readily recognizable. The mesenchyme is less compact than 

 hitherto. The Anlagen of the muscles are particularly well 

 marked out as prominent patches of densely aggregated mesen- 

 chyme cells, containing numerous yolk-spherules. The blood- 

 vessels also have begun to form in the head region. 



PI. VI, figs. 2, 3 and 4 are coronal sections of a tadpole of this 

 stage. Of these fig. 2 was taken at a plane a slight distance above 

 the floor of the pharynx. Comparing it with fig. 1 we find that 

 anteriorly the stomatodeal invagination (d.) has deepened very 

 considerably, although as yet not communicating with the pharyn- 

 geal cavity. The latter has much the same form as in fig. 1, 

 except that posteriorly an additional visceral-cleft is present. Of 

 these clefts the most anterior, the hyomandibular (Stjm;), can be 

 seen as a narrow, solid diverticulum of the pharyngeal wall, 

 extending outward and terminating bluntly in the mesenchyme a 

 short distance below the external ectoderm. All the remaining 

 cleft outgrowths reach to and blend with the external ectoderm, 

 although as yet not opening to the exterior. Within the body of 

 each of the two anterior visceral-arches— i.e., mandibular and 

 hyoid — the muscles can be made out as ixTegularly defined patches 

 of denser mesenchyme. That in the mandibular arch is the 

 Anlage of the muscles of mastication (^k.m.), while that in the 

 hyoid arch is the Anlage of the depressor mandibulse + depressor 

 ossis hyoidei' {h.m.). Anterior to the first cleft is a small vessel, 

 the mandibular aortic arch (m.a.), while on the left of the figure 

 another vessel is to be seen posterior to the cleft. The latter is the 

 hyoidean aortic arch (/i.«. ). 



PI. VI, fig. 3 is taken at a considerably higher level. On the right 

 side we have passed above the dorsal margins of the visceral-clefts, 

 so that the latter are shown only on the left side. This section 

 passes in a plane approximately on a level with the base of the 

 brain, the small dark patch in the median hue in front of the 

 pharyngeal cavity being the floor of the iufuudibulum (inf.). 

 The hyomandibular fold can be seen extending outward and slightly 

 forward. It will also be noticed that its distal* end approaches the 



' Spemann includes these two muscles under the term " orbito-hyoideus. ' 

 ® A word of explanation is necessary concerning my use of the terms "dis- 

 tal" and "proximal." Ordinarily these terms are used only in connection 



