22 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 5 1 



figure gf, shows a marked change in the form of the embryo. While 

 of about the same lateral dimensions, the dorso-ventral diameter of 

 the embryo in this region is less than one-half what it was in the 

 head region. The epidermal ectoderm (ep) is now nearly horizontal 

 in position and is not so abruptly separated laterally from the thin 

 lateral sheets of ectoblast. The medullary groove (mg) is here a 

 very narrow vertical slit. At this stage the fusion of the medullary 

 folds has taken place over the anterior third of the embryo. For a 

 short distance, represented in about thirty-five sections, the canal is 

 open, as in the figure under discussion ; for the next one hundred 

 sections (about one-third the length of the embryo) in the region of 

 the mesoblastic somites the canal is again closed, while throughout 

 the last one-third of its length the canal is widely open dorsally. The 

 enteron is here entirely open ventrallv, the entoderm being almost 

 flat and horizontal. The notochord (nt) is distinctly outlined and is 

 somewhat flattened in a dorso-ventral direction. The body cavity 

 (be) is well marked, but is separated by a considerable mass of un- 

 cleft mesoblast from the notochord and the walls of the medullary 

 groove. 



A space of about one hundred sections, or one-third the length of 

 the embryo, intervenes between figures gg and 91. This is the 

 region of the mesoblastic somites, and in this region, as has been 

 above stated, the medullary canal is completely enclosed. It is evi- 

 dent then that the entire anterior two-thirds of the medullary canal 

 is enclosed except for the short region represented in figure Sg. 

 Whether or not this short open region between the two longer en- 

 closed regions is a normal condition the material at hand does not 

 show. 



Figure gh represents a typical section in the region of the meso- 

 blastic somites just described. It shows the enclosed medullary 

 canal (mc), the body cavity (be) on the right, and a mesoblastic 

 somite with its small cavity (myc) on the left. The entire section 

 is smaller than the sections anterior or posterior to this region, and 

 seems to be compressed in a dorso-ventral direction, this compres- 

 sion being especially marked in the case of the notochord. 



Figure 91 is through a region nearly one hundred sections pos- 

 terior to the preceding, and cuts the embryo, therefore, through the 

 posterior one-fourth of its length. The chief difference between this 

 and the preceding section is in the medullary canal, which is here 

 open and is in the form of a wide groove with an irregular, rounded 

 bottom and vertical sides. The size of the section is considerably 

 greater than in the preceding, the increase being especially noticeable 



