4<> SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 5 1 



Figure i6g represents a sagittal section of the anterior half of 

 the body of an embryo of this or possibly a slightly younger stage 

 of development. The three regions of the brain are clearly indi- 

 cated, as well as the cavity of the spinal cord (sc). The roof of the 

 hindbrain has been made too thick in the figure ; it should be rep- 

 resented by a mere line. A little mesoblast is to be seen at places 

 between the roof of the brain and the superficial ectoderm. A slight 

 invagination of the epithelium (p), between the floor of the brain 

 and the anterior end of the notochord, probably represents the begin- 

 ning of the hypophysis. Xo indication of the pineal body is yet 

 to be seen. Extending from the region of the hypophysis to the 

 posterior end of the section is the notochord (nt) ; it is much vacuo- 

 lated and gradually increases in thickness toward the posterior, 

 though its outline is quite irregular; except at the extreme anterior 

 end and at one or two other places, it lies in close contact with the 

 floor of the neural tube. Directly under tin- notochord lies, in the pos- 

 terior half of the figure, the large dorsal aorta (ao). The pharynx 

 (ph). opening between the end of the forebrain and the thick man- 

 dibular fold (across which opening the amnion (a) of course ex- 

 tends), is a funnel-shaped space which passes out of the plane 

 of the section toward the posterior end of the figure. Its thick 

 endodermal lining extends to the mandibular fold on the ventral 

 side, while on the dorsal side it gradually thins out and becomes 

 continuous with the thin ectoderm that extends over the forebrain. 

 Just back of tin- mandibular fold is the bulbus (b), and back of that 

 is the edge of the ventricle (vn). Posterior and dorsal to the ven- 

 tricle the liver ( li ) is seen as an irregular mass of cells, and dorsal 

 to the liver one of the Wolffian bodies (wt) is cut through its ex- 

 treme edge. 



Stack XIV 

 Figures 17-17^ (Plates XVIII, XIX) 



Body flexure has increased until now the forebrain and tail are 

 almost in contact (fig. 17). The eye has developed somewhat; the 

 ear vesicle, which is not shown in the figure, is small and seems to 

 lie nearer the ventral side; the nasal pit is much larger and is 

 crescentic in shape. The hyomandibular cleft (g') still persists as a 

 small crescentic slit, while the next three clefts are now represented 

 merely by superficial grooves separated by distinct ridges, the vis- 

 ceral folds. No indication of a fifth cleft is seen. The maxillary 

 process (m.v) grows ventralward under the forebrain and is already 

 longer than the manibular arch (md). 



The chief advance in development over the preceding stage, be- 



