NGALLS. 



PLATE 3. 



*' 



' 





. '..-^F 



x 



V 



3. 



4 

 .j. 



d .">. The following drawings (indicated l>y I> in the text-figures) arc obviously ivronMi unions ami therefore >ome- 

 wluit schcnuilic. They \voulil cut the mc.liaii line at or near the points travelled \\\ the sections shown 111 llie 

 liholonraplis (Plate 2, tigs. 1 to 4), and have therefore been ilesignateil by the same section number. In othei- 

 won Is section (photograph I 4L'."> and drawing (section) 42.") would intersect in the median line. Like the photographs 

 they are slightly oblique dorsoventrally . 



(Section) 4l'.~>. I'oslerior part of primitive streak. X 100. 



iSectionl 4()(i +. Iinrsal upening of archenteric canal. X 100. 



(Section) 401. Typical head process, archenteric canal, "plaque notocliordale" and "plaque Iccithoentcrique." X 100. 



i Sect ion i *>4 +. Large venlral opening, "plaque notoehordale." X 100. 



Idealized cross-section 1 1 ('construction ) at the point where all three genii-layers are continuous. Its location, while 

 not indicated, can lie made out from text-ligure .">. 



