226 PROCEEDINGS OF THE ACADEMY OF [March, 



Between the visceral- clef ts intervene the visceral-arches. The 

 interior of each arch is made up of a mass of rather compact 

 mesenchyme, consisting of scattered cells, containing numerous 

 large yolk- spheres, barely distinguishable from those occurring in 

 the endodermic lining of the pharynx. From this circumstance 

 the limits of the endoderm are somewhat difficult to define clearly, 

 and accordingly considerable care had to be taken in outlining it. 

 The endoderm is, however, much more densely crowded with yolk- 

 spheres and hence appears as a darker layer more or less clearly 

 marked off from the surrounding lighter mesenchyme. Four 

 visceral-arches are clearly differentiated, the two anterior of which 

 are the mandibular (k.m.) and hyoid (h.in.) arches, while the 

 other two are the tirst and second branchial-arches. In the former 

 two a somewhat dense patch of mesenchyme can be seen occupying 

 the centre of each. These patches are the Anlagen of the future 

 muscles of these arches {h.m. and h.m.). 



An examination of the remaining sections of the series to which 

 fig. 1 belongs, shows that the pharyngeal cavity retains approxi- 

 mately the same size throughout its entire dorse-ventral extent 

 and that throughout their entire length the visceral-clefts have 

 about the same direction and relations as shown in the figure. 

 Hence we may look upon the clefts as being solid folds of endo- 

 derm, compressed antero-posteriorly and elongated dorso-ventrally. 

 Throughout their entire extent the first three clefts arc apparently 

 in contact with the deeper layer of the ectoderm. 



The first or hyomandibular cleft resembles the other clefts in all 

 essential respects, except that it extends slightly forward whereas 

 the second extends transversely outward, while the remaining two 

 course obliquely backward. A section of the cleft in almost any 

 coronal plane presents the condition shown in the figure. Imme- 

 diately dorsal to the outer extremity of the cleft the distal portion 

 of the facial ganglion becomes continuous with the deeper or 

 sensory layer of the ectoderm. 



Stage II (Pis. VI, VII, figs. 2-7).— In this stage all five vis- 

 ceral-clefts are present, none of which opens to the exterior. The 

 mouth is still separated from the pharynx by the stomatodeal plate. 

 The external gills have budded forth as two minute, blunt, undi- 

 vided processes from the sides of the first and second visceral- 

 arches. 



