84 MCMURRICH. [Vol. XI. 



When the concentration is complete the mesoderm forms a 

 somewhat oval mass of cells whose anterior edge is surrounded 

 by a row of ectodermal cells which are the most posterior cells 

 of the ectodermal plate. Injacj-a on surface view these cells 

 are not specially differentiated from the other ectodermal cells 

 of the plate, but the further development shows that they 

 possess a somewhat special function, and that they correspond 

 to the ectodermal teloblasts which become evident later on, 

 and which have been described in CymotJioa by Patten ('90). 

 They now begin to divide in a direction at right angles to the 

 long axis of the embryo, and by repeating this division give 

 rise to rows of cells extending forward from them. By this 

 process the teloblasts are themselves forced backward over the 

 mesendodermal plug, which thus becomes covered by ectoderm. 

 About the time, however, that the mesendodermal plug is 

 about half covered, the vitellophag cells, which, 'up to this 

 time, have retained their original position, begin to migrate 

 into the interior of the yolk, as is shown in the section repre- 

 sented in PI. VI, Fig. 22. In this section one sees anteriorly a 

 row of ectoderm cells {ec) which represent the ectodermal 

 plate, and behind them two cells {tt) which form one of the 

 rows of cells produced by the division of the teloblast {T). 

 This cell appears considerably larger than the other ectodermal 

 cells, but so great a difference as occurs in this particular case 

 is not as a rule noticeable. Below the teloblast and behind it 

 are found a number of mesoderm cells {MEn) representing the 

 mesendodermal plug, but which are much more loosely aggre- 

 gated than they were in the section represented in Fig. 21. 

 The chief interest of the section, however, lies in the arrange- 

 ment it shows for the vitellophag cells {vi). Two of these ar-e 

 seen ; one still occupies a position at the surface, while the 

 other has sunk into the yolk a short distance. The example 

 of the latter is followed later by the other vitellophags, and it 

 may be again remarked that when they begin their immigration 

 there is no trace of cells in the interior of the yolk, nor have I 

 succeeded in finding evidence that any of the vitellophags 

 come from other regions of the blastoderm. In later stages 

 the vitellophags are certainly more numerous than they were 



