42 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 5 1 



cord being even more marked than is shown in the figure. ( )n 

 either side of the spinal cord a large spinal ganglion (sg) is sien, 

 closely wedged in between the spinal cord and the adjacent muscle 

 plate ( mp ) . As in the preceding stage, there is a marked space 

 between the muscle plate and the adjacent ectoderm (ec). The 

 somatic mesoblast at the upper angle of the unenclosed body cavity 

 is thickened on each side and somewhat bulged out by the Wolffian 

 body to form what might be termed a Wolffian ridge (wr). In the 

 mid-ventral line is the considerably developed mesentery ( ms ) , 

 from which the intestine has been torn. The Wolffian bodies now 

 consist, on each side, of a group of five or six tubules (wt) of 

 various sizes, near which in a more ventro-lateral position, close to 

 the upper angle of the body cavity, is the more distinct Wolffian 

 duct (wd). The allantois is fairly large by this time, and may be 

 seen in the most posterior sections as an irregular, thick-walled out- 

 growth from the hindgnt. 



A horizontal section through the anterior end of an embryo of 

 this age is shown in figure 15/. Wdiile enclosed of course in the 

 same membranous amnion (a), the pharyngeal region of the section 

 is >eparated by a considerable space from the more anterior region 

 where the section passes through the forebrain (fb) and eyes. The 

 spinal cord (so, notochord (nt), muscle plates (mp), aorta; (ao), 

 and anterior cardinal veins (ac) need no special description. The 

 appearance of the pharynx [pit), with its gill clefts and folds, is 

 quite similar to that of the corresponding structures in the chick. 

 None of the four clefts (g 1 ' 4 ) show, in the plane at which the sec- 

 tion was cut, any connection with the exterior; in fact the fourth 

 cleft (g*) would scarcely be recognized as a cleft if seen in this 

 section alone. One or two of the more anterior clefts are open to 

 the exterior. Three pairs of aortic arches are seen, and each vis- 

 ceral fold has a central condensation of mesoblast. 



Stage XIII 



Figures i6-i6g (Pi.atks XVII, XVIII) 



The embryo (tig. 16) now lies on one side, body torsion being 

 complete. The curvature of the body is so marked that the exact 

 length is difficult to determine. The eye (c) and ear ( o) have 

 about the same superficial appearance as in the preceding stage. 

 The nose is not shown in this figure. About thirty somites are 

 present; the exact number cannot be determined in surface view. 

 The amnion is complete, though not shown in the figure, and the 

 tail (/ ) is well formed. The umbilical stalk was torn in the removal 



