20 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 5 1 



nuclei. Where the ectoderm becomes thickened to form the medul- 

 lary folds, the cells are much more compactly arranged; hence this 

 region stands out in strong contrast to the rest of the ectoderm. 



Stage VI 

 Figures ga-gm (Plates IX, X) 



The embryo represented by this series of transverse sections is 

 intermediate in development between those represented in surface 

 views by figures 8 and 10. The amnion and head-fold are nearly the 

 same as in figure 8; the medullary folds are intermediate in devel- 

 opment, the anterior end not showing so marked an enlargement as 

 shown in figure io, v' '. There are six or seven faintly distinguishable 

 somites. 



Figure ga represents a section through the anterior part of the 

 head-fold ; it shows one unusual condition : the head lies entirely be- 

 neath the surface of the yolk. This condition is quite confusing 

 when the section is studied for the first time. The pushing of the 

 head under the yolk is shown at its commencement in figure II. 

 The process continues until nearly the entire anterior half of the 

 embryo is covered ; but when the embryo attains a considerable size 

 it is seen to lie entirely above the yolk, as in the chick. According 

 to Voeltzkow's figures (18), this same condition is found in the 

 crocodile, and Balfour (2) also mentions it in connection with the 

 lizard. The fusion of the medullary folds has made considerable 

 progress, so that the entire anterior end of the canal is enclosed, 

 except in the region where the folds are bent down and back, as in 

 the preceding stage ; here the folds are still distinct from each other, 

 leaving the medullary canal open on the ventral side, as shown in 

 figures 9 and cjb. In the section under discussion the ectoderm (cc) 

 is a very thin membrane on top of a considerable mass of yolk, while 

 no entoderm can be distinguished. The amnion (a) completely sur- 

 rounds the embryo as an irregular membrane of some thickness in 

 which no arrangement into layers can be seen. The epidermal ecto- 

 derm (cp) is composed of the usual loosely arranged cells, so that it 

 is clearly distinguishable from the compactly arranged cells of the 

 nervous layer («/), from which it is separated by only a line. 



In figure gb, which shows a section a short distance posterior to 

 the preceding, the medullary canal (mc) is somewhat deeper and is 

 still open ventrally. There is a distinct space between the nervous 

 (nl) and epidermal (cp) layers of the ectoderm, in which space a 

 few mesoblast cells (mes) may be seen. The section is cut just pos- 

 terior to the edge of the amnion, so that there is now neither amnion 

 nor yolk above the embryo. 



