EXPLANATION OF PLATES IV AND V, 



Illustrating the Memoir on some Points in the Early 

 Development of the Common Newt [Triton taniatus)^ 

 by W. B. Scott, B.A., and Henry F. Osborn, B.A. 



"With the exception of fig. 1 the following figures were drawn with a 

 Zeiss' A objective. lu figs. 2, 3, 4, 5, a No. 2 (Zeiss) eyepiece was used, 

 aud for figs. 6 and 7 a No. 3 eyepiece. 



EXPLANATION OF PLATE IV. 

 List of Heferences. 



ep. Epiblast. (?;/- Inner layer of epiblast. yJc. Yolk. hy. Hypo- 

 blast, in. hy. Invagination hypoblast. y. hy. Yolk hypoblast. m. 

 Mesoblast. sp. Splanchnopleure. so. Somatopleure. al. Alimen- 

 tary canal. nc. Neural canal. ch. Notochord. mg. Medullary 

 groove. mf. Medullary folds. 



Fig. 1. — Longitudinal section of an embryo at time of commencement of 

 invagination. Hartnack No. 7 obj., eyepiece 3. It shows one of the 

 earliest stages of the epiblast. 



Fig. 2. — Represents a longitudinal section of a Triton embryo (probably 

 cristatus) in the early part of Stage a. At the opening of the blastopore 

 the section is in the median line. It slants off forwards, however, to one 

 side, and therefore out of the region of the alimentary canal. It shows 

 the formation of the invagination-hypoblast and the confused mass of cells 

 arising from the reflection of the epiblast. 



Fig. 3. — A section of the same embryo. It may be considered the re- 

 verse of the last. At the blastopore it is at one side of the median line, 

 while anteriorly it is directly in the median line. This obliquity explains 

 the apparent upgrowth of yolk-cells in the centre. Putting this and the 

 previous section together, a fair idea may be obtained of the actual relation 

 of the layers at this period. It illustrates the formation of mesoblast 

 by invagination, and the obliteration of the segmentation cavity by 

 the advance of the alimentary canal. The blastopore has been artificially 

 widened. 



Fig. 4. — An anterior transverse section of an embryo, at Stage a, slightly 

 more advanced than the previous one. It shows the shallow medullary 

 groove, the lateral plates of mesoblast extending half way down the sides, 

 also the invagination-hypoblast above the alimentary canal continuous at the 

 sides with the yolk hypoblast. 



Fig. 5. — A transverse section through the head region of an embryo of 



