68 CONK UN. [Vol. XIII. 



mesoblast is developed. Since this cell gives rise to the pos- 

 terior part of the alimentary canal as well as to the mesoblast, 

 I shall call it the mese7itoblast, ME {= 4d). 



Soon after its formation it divides, as shown in Fig. 25, into 

 right and left halves, ME^ and ME^ ; this division is dexio- 

 tropic, as is shown by the fact that the right half overlaps the 

 left. Figs. 26 and 27. These cells remain for some time 

 in the position in which they are formed ; they lie to the 

 right of the future median plane, which is marked by the second 

 cleavage furrow, and are more nearly symmetrical with refer- 

 ence to the ectoblastic plate than to the macromeres, Figs. 26, 

 29, 30. The next cleavage of these cells. Fig. 30, leads to the 

 formation of 



2. The Prhnary Enterobiasis . Figs. JO, J/. 



The spindles which introduce this division are bilaterally 

 symmetrical with reference to the line along which the two 

 cells are in contact ; anteriorly the spindles diverge from this 

 line and at the same time slant upward, so that the cells which 

 are given off anteriorly lie at a higher level than the posterior 

 moieties. Fig. 3 1 . These anterior cells are about equal in size 

 to the posterior ones, but contain less yolk. The posterior 

 cells are the primary enterobiasis, and together with two other 

 cells, to be described in a moment, give rise to the posterior 

 or distal end of the intestine. They are purely entoblastic, 

 and do not divide again until about the time of the closure of 

 the blastopore. The anterior cells are still of mixed character, 

 containing both mesoblast and entoblast. 



Up to the last cleavage there had not been a single bilateral 

 division ; even in the formation of the mesentoblast and its 

 division into right and left halves, all the cleavages were spiral. 

 But with the division of the right and left mesentoblasts, by 

 which the primary enteroblasts are cut off posteriorly, bilateral 

 cleavages suddenly appear. All subsequent divisions of the 

 mesoblast, as far as I have been able to follow them, are 

 bilateral. In the ectoblast and entoblast, however, bilaterality 

 appears very gradually, and is not prominent until a very late 

 period. 



