134 whitman: [Vol.1. 



than the outer faces. Fig. 83 represents a horizontal section 

 of the stage of Fig. 37, which passes beneath the neuroblasts 

 and the blastodisc. Here only two nuclei were hit, but these 

 lie near the outer faces of the blastomeres. The nuclei of the 

 hlastomeres then pass from their original central position to the 

 periphery, and can here be seen in the living egg." (No. I, 

 pp. 57-58.) 



I was unable to trace these nuclei directly through all their 

 later stages of multiplication, but various facts led me to con- 

 clude that they gave rise to the mesenteron. Between the last 

 stage in which I could recognize these nuclei and that in which 

 the mesenteron became distinct, a number of stages intervened 

 in which I was unable to demonstrate their existence. This 

 failure, due to imperfections in methods of preparation, was a 

 source of doubt, in spite of the many indications which made 

 it appear almost certain that they represented the mesenteron. 

 I satisfied myself that the entoderm could not have its origin 

 in elements derived from the germ-bands, for all these elements 

 were plainly turned to other uses. Besides, the earliest ap- 

 pearance of the entoderm cells, — their loose and irregular 

 order in the periphery of the yolk, — pointed to their origin 

 from the free superficial nuclei of earlier stages. After describ- 

 ing the earliest appearance of the mesenteron (p. ^6^, I stated 

 my conclusion in the following words : — 



" These superficial nuclei go on multiplying by division dur- 

 ing the whole period of the epiboly. Finally they are seen as 

 mere white dots scattered over the entire surface of the yolk. 

 Six to seven days after exclusion the entoderm cells make their 

 appearance as clear cells with small nuclei, in the periphery of 

 the yolk already cut up into compartments by the septa. 

 What hypothesis is more probable than that these cells origi- 

 nate from the free nuclei? My sections have convinced me 

 that these entoderm cells arise in the surface of the yolk, and that 

 they do not originate in the products of the blastodisc" (p. 6^^. 



The condition of the mesenteron represented in Figs. 93 and 

 95 of my former paper, is very similar to that shown in Nus- 

 baum's Figs. 13 and 14, PI. II. 



With regard to the origin of the free nuclei from the primary 

 nuclei of the entoblasts, I have nothing to add to the state- 

 ments above cited. The observations which follow begin with 



