524 STANLEY C. BALL 



filled with a plasma. To how great an extent the separation 

 of the yolk-filled contents into two zones is due to the action of 

 reagents has not been determined. It is interesting to note 

 in this connection that most of the spherical yolk-mass enclosed 

 in the embryo in figure 42, an Ehrhch-stained specimen, is 

 homogeneously filled with yolk-granules such as originally 

 entered the cells. Still this embryo, as indicated by the presence 

 of the cilia, is more advanced in development than the one 

 illustrated in figure 34. 



One entoderm cell, that at the upper left in figure 42 demands 

 attention. The yolk material which it contains has entirely 

 assumed the fluid nature seen in central regions of these cells 

 (fig. 36). Now it has been explained that the Flemming-iron- 

 haematoxylin-Bosin method leaves these regions entirely color- 

 less. Hence the cell now under consideration, if stained by 

 this latter method, would have apparently contained a vacuole 

 in the position here occupied by the yolk mass. Exactly such 

 a condition is exhibited in figure 37. In this embryo there 

 appear what at first were regarded as vacuoles within a mesen- 

 chymatous network; they are now rather to be interpreted as 

 the transformed yolk material accumulated within the entoderm 

 cells as immense globules which, following the corrosive-acetic 

 Ehrlich's haematoxyhn-eosin method, stain a rich yellow. 

 It has been pointed out that by this method also the nuclei, 

 cytoplasm and membranes of the entoderm cells containing 

 these nutritive masses are much more clearly stained than by 

 the iron-haematoxylin. 



Another feature to be noted in the entodermic yolk masses in 

 figure 42 is that the mitochondrial mass is again discernible. 

 This body was traced from its origin within the oocyte through 

 cleavage to its division and enclosure in the primary entoderm 

 cells. During the early history of the mass the Ehrlich stain 

 was unfavorable to its demonstration, since the cytoplasm of the 

 cell retained so deep a. stain that the mitochondrial mass was 

 obscured. But in the late stage shown in figure 42 the cytoplasm 

 of the original cell has broken down and the granular mass of 

 mitochondria again comes into prominence. The centrosome 



