-2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 5 1 



Stage XVII 

 Figures 20-20/ (Plates XX. XXI) 



The superficial changes noted in this stage chiefly concern the 

 head, which has increased considerably in length (fig. 20). The 

 curvature of the body is slightly more marked, and the tail is more 

 tightly coiled at the end. There are still signs of three gill clefts. 

 The maxillary process (m.r) is long and narrow, while the mandibu- 

 lar arch ( md) is still short and broad. The fronto-nasal region has 

 greatly increased and has the acquiline profile noted by Clarke. The 

 nasal groove has disappeared, and there remains the small opening 

 (11) at the side of the fronto-nasal region, near the end of the still 

 separate maxillary process. The umbilicus is in about the same 

 condition as in the preceding stage, but the heart is less prominent. 

 The outline of the manus {ma) is more definite, and the extremity 

 of the posterior appendage is distinctly flattened out to form the 

 rudimentary pes (pe). The position of the elbow-joint in the an- 

 terior appendage is seen at the end of the reference line aa. 



Typical transverse sections of this stage are shown in figures 

 20a- j. 



Figure 20a is a section through the middle region of the head, 

 cutting the hindbrain on one side and the forebrain on the other. 

 The walls of the brain show rather more histological differentiation 

 than was seen in the preceding sections, though this cannot be 

 shown under the low magnification used. The hindbrain (///'). 

 which is cut near its anterior border, exhibits the usual membranous 

 dorsal and thick ventral walls. The forebrain is here seen as three 

 distinct cavities — a median third ventricle (vt), with a thick ventral 

 wall, and a thin dorsal wall extended to form a large pineal body 

 (epi), and two lateral ventricles (<"/;). the cavities of the cerebral 

 hemispheres, whose walls are quite thick except on the side next the 

 third ventricle. The sections of this series being slightly oblique, 

 the eye is here cut on the right side only, where it is seen as a large, 

 semicircular cavity (<?) with thick, dense walls. The mesoblast. in 

 which several blood-vessels (bv) are seen, exhibits three distinct 

 areas — a median, lighter /.one. with a more dense ana on either 

 side. The significance of this variation in the densit) *<i the meso 

 blast is not apparent. 



Figure 20b is only a few sections posterior to the section just 

 described. It is drawn chiefly to show the appearance of the fore 

 brain, the other structures being about as in the preceding figure, 

 except that both eyes (e) are here represented. The section passes 

 through the wide opening between the third (/;) and the lateral 



