6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 56 



with columnar epithelium outside of which is a dense layer of meso- 

 blast continuous with the mesentery. In the center of the figure the 

 allantois, al, is seen as an irregular cavity, lined with a single layer of 

 columnar or cuboidal cells, and surrounded by a thick mass of 

 loosely arranged, stellate mesoblast cells. The allantois is probably 

 somewhat larger here than in the other embryos used for this stage, 

 in which it was torn away. The tail, t, of the embryo is shown at the 

 lower side of the figure, surrounded by the amnion ; it is cut in the 

 region of a curve so that the caudal intestine, i, is cut longitudinally 

 and has the outline of an elongated ellipse. In this embryo the 

 caudal intestine could be followed to the end of the tail, through 

 several dozen sections ; for some distance posterior to the allantois 

 it is extremely narrow, so that its lumen is almost obliterated, and its 

 walls are made up, in any one place, of not more than a dozen cu- 

 boidal cells. Towards the posterior end of this region the intestine 

 is considerably enlarged as seen in figure 4L. 



Figure 4M passes through the region where both the allantois and 

 the Wolfi[ian ducts open into the hindgut. The union of the allantois 

 and the gut accounts for the elongated outline of the enteron in this 

 section. The openings of the Wolffian ducts, zvdo, are seen at the 

 lower end of the section of the enteron. The cells lining the Wolffian 

 ducts are smaller than those lining the enteron. In the lower side 

 of the figure are seen the structures of the tail, including the outline 

 of the tiny caudal intestine, i, mentioned above. No sign of a cloacal 

 invagination could be made out with certainty. 



The next stage to be studied is shown in surface view in figure 5. 



Figure 5A represents a section through the head region of this 

 embryo. Owing to the obliquity of the plane of the section the figure 

 is quite asymmetrical. The pharynx, ph, is lined with a compara- 

 tively thin epithelium and opens, on the left, at two places, one the 

 mouth and the other the second gill cleft, g'. In the dorsal wall of 

 this cleft, as well as in the corresponding wall of the opposite cleft, 

 is seen a thickening of the epithelium ; these thickenings, ty, are the 

 rudiments of the thymus gland, whose development may be described 

 in detail in another paper. Compared to the size of the gill clefts 

 the cavity of the pharynx is, at this stage, comparatively small. 



Followed caudad the pharynx becomes depressed until, in the 

 region shown in figure 5B, it is a mere narrow slit, g, extending 

 transversely across the embryo and opening through the gill clefts 

 to the exterior on each side. 



Figure 5c passes through the posterior region of the pharynx, ph, 

 the tip of the forebrain, fh, the anterior edge of the heart, ht, and 

 the curve of the tail, /. The chief point of interest in this section is 



