PLATE 4 



EXPLANATION OF FIGURES 



40 and 41 Selected sections of a Q.5 mm. embryo. In figure 40 the clear proto- 

 plasm at the extreme tip might be interpreted, but for the limiting membrane, as a 

 part of the ectoderm, it conforms so perfectly to the contour of that layer; while 

 the ectoderm seems hollowed out for its reception, the clearly defined line 

 makes it easy to distinguish the layers. Figure 41 shows the ectoderm as if 

 grooved for the reception of the duct. These sections show the ectoderm much 

 thickened and the cells placed so that the nuclei have their axes at right angles 

 with that of the layer. 



42 to 47 Cross sections of a 11 mm. embryo, showing much the same features 

 as the last, except that the separation of the duct from the ectoderm is more abrupt 

 and the duct is rounded and more regular. The ectoderm is less modified and 

 much thickened, with the cells shorter. 



48 The anlage of the duct where it reaches the cloaca in a 20 mm. embryo. At 

 this point the duct bends sharply inward to reach the cloaca, hence the section is 

 oblique. The wall of the duct is of columnar epithelium. It has not yet broken 

 through to the cloaca, but the walls are fused, and the drawing shows the duct as a 

 triangular loop attached to the cloaca, c, by its base. 



370 



