ji, SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 5 1 



In figure i lb is represented a section, behind the preceding, which 

 passes through the posterior tip of the turned-under anterior end 

 (mc'). Here the medullary canal is closed both above (mc) and 

 below (mc'). The amnion (a) has about the same appearance as in 

 the more anterior section, but there is here a considerable space, 

 filled with mesoblast (mes), between the nervous (nl) and epidermal 

 (ep) layers of ectoderm. 



Figure uc is twenty sections, about one-tenth the length of the 

 embryo, posterior to the one last described. The large mass of over- 

 hanging yolk (y) is still present, as is also the amnion (a), though 

 the latter no longer passes entirely around the embryo ; only the true 

 amnion could be made out. The thickened walls of the medullary 

 canal have reduced that cavity to a narrow, Y-shaped slit (mc). 

 The notochord (nt) is very slender in this region, compared to its 

 diameter farther toward the posterior end. The enteron (ent) is a 

 large cavity, whose wall is made up of loosely arranged cells except 

 around a median, ventral depression where the cells are more com- 

 pact. This depression may be traced through ten or fifteen sections 

 and may represent the beginning of the thyroid gland, though this 

 point was not worked out with certainty. Surrounding the noto- 

 chord and enteron is a loose mass of typical, stellate mesoblast cells 

 ( mes), which are cleft on either side to form the anterior limit of 

 the body cavity (be). Between the bod}- cavity below and the en- 

 teron above, on each side, is a small blood-vessel (bv) which when 

 followed caudad is found to open ventrally and medially into the 

 anterior end of the heart. 



Figure lid is about a dozen sections posterior to the preceding. 

 The appearance of the overhanging yolk (y) of the amnion (a) and 

 of the notochord (nt) is about as in the more anterior section. The 

 medullar}- canal (mc) is a straight, vertical slit, and the depression 

 in the floor of the pharynx | cni ) is much more shallow. The body 

 cavity (be) is much larger and extends across the mid-ventral line 

 beneath the heart (ht), which is cut through its middle region. The 

 heart may be traced through about twenty sections (one-tenth the 

 length of the embryo) ; its mesoblastic wall (mes') is thin and irreg- 

 ular, and is lined by a distinct endothelium (en) whose exact origin 

 has not yet been worked out. 



Figure lie is just back of the heart, and shows in its place the 

 two vitelline veins (vv). The depression in the floor of the enteron 

 (ent) is entirely distinct from the one that has been mentioned 

 above, and is simply the posterior limit of the head-fold of the ento- 

 derm ; the fifth section posterior to this shows where this depression 



