2O6 



S. J. HOLMES. 



Should they be gill filaments they would afford a case of the self 

 differentiation of an organ in the absence of certain stimuli, such 

 as those afforded by the blood supply, which normally are 

 associated with their formation. 



Fig. 2 represents a development from a cross section of a late 

 embryo near the middle of the tail region. This piece, like the 

 one just described, was at first opaque, owing to the large amount 

 of yolk in the cells. A week later it became quite transparent, 



FIG. 2. A cross section from near the middle of the tail of a frog embryo drawn 

 seven days after being cultivated in lymph. 



and numerous pigment cells became differentiated beneath the 

 ectoderm. Externally it was completely covered by a layer of 

 flattened epithelial cells. The connective tissue within the piece 

 had become more specialized. At either end of the piece there 

 appeared a prominent outgrowth, the one at the posterior end 

 curving upward and forward. There were two large cavities 

 lined with a single layer of much flattened endothelial cells. 

 These cavities were much larger than any spaces in the original 

 piece of the embryo, and not improbably represent closed and 

 distended blood vessels. The notochord showed at each end an 

 extension of cells which suggested that the organ was undergoing 

 regeneration in both directions. These extensions lay entirely 

 within the new outgrowths from the cut ends of the piece. 



Fig. 3 represents a developed fragment taken five days pre- 

 viously from near the middle of an embryo. This piece contained 

 some entoderm which is included in the dark pigmented mass 



