34 J- B. JOHNSTON. 



FIG. I. Transverse section through the ventral part of a young embryo immedi- 

 ately behind the site of the mouth, to show the area of growth in the entoblast pre- 

 paratory to the formation of the heart endothelium. The dotted circle indicates the 

 position in which the foregut appears in the next section forward. 



FIG. 2, a, b, f, e. Transverse sections nos. 212, 224, 235, 251 of an embryo in 

 which the first gill slit has just made its appearance. Sections IO microns thick. 



FIG. 3, a, b, c, d, e. Transverse sections nos. 273, 292, 306, 318, 325 of a later 

 embryo in which the separation of the endothelial cells from the entoblast is nearly 

 completed. Sections IO microns thick. The sections shown in Fig. 3, a, b, c, t are 

 approximately at the same levels as those shown in Fig. 2,a,b,c,e, respectively. 



FIG. 4. Median sagittal section of the region between the mouth and liver of an 

 embryo of the same age as that shown in Fig. 3. Cell boundaries are shown 

 wherever they can be seen. The heart endothelial cells are evidently continuous 

 with the entoblast behind the mouth, but independent at all other points. 



All figures were drawn with Zeiss apochromatic lenses and camera. Figures I, 2, 

 and 3 were drawn with 16 mm. objective and no. 4 ocular ; Fig. 4 with 8 mm. 

 objective and no. 4 ocular, and all have been reduced to one third in reproducing. 



BIBLIOGRAPHY. 

 Brachet. 



'98 Developpment du coeur chez les Amphibiens urodeles. Archives d'Anatomie 



micr., T. 2, p. 251-304, 1896. 

 Morgan. 



'97 The Development of the Frog's Egg, an Introduction to Experimental 

 Embryology. New York, 1897. 



Ralb. 



'86 Ueber die Bildung des Herzens der Amphibien. Morph. Jahrb., Bd. 12, pp. 



252-273. 1886. 

 Schwink. 



'91 Untersuchungen iber die Entwickelung des Endothels und der Blutkorperchen 



der Amphibien. Morph. Jahrb., Bd. 17, pp. 288-333. l8 9 I - 

 Ziegler. 



'02 Lehrbuch der vergleichenden Entwickelungsgeschichte der niederen Wirbel- 

 tiere. Jena, 1902. 



