592 HAROLD HEATH, 



pointed and their apices ultimately meet the advancing tip cells. At 

 first the contact surface between these cells is small, but in a short 

 time it grows broader and becomes as great as the smaller diameter 

 of the cells. Thus by movements of the tip cells and primary trocho- 

 blasts the lower row becomes complete. 



In the second quartette of the posterior quadrant in the early 

 stages the divisions undergone are much the same as those of the 

 other quadrants, but generally commencing about the time of the 

 division of the tip cell changes occur which cause these latter cells 

 to lie below the line of the prototroch and consequently a gap is 

 produced as wide as the original tip cell. 



The movements whereby the upper row of cells is completed are 

 similar to those of the remaining quadrants. They extend themselves 

 across the gap between the upper cells of the somatic plate and the 

 median cross cells lying above, and ultimately come in contact. The 

 contact surface however always remains of smaller area than in the 

 other cases (e, diagram B). 



The lower row cells all become flattened from above downward, 

 increasing their long diameters, and the cells Id'-^-'^-i and 7c-^-^-^^ 

 assume a wedge-shaped form similar to that of the cells of the upper 

 row. They come in contact with the tip cells, but as these gradually 

 sink into the lower hemisphere they continue to flatten and the apex 

 to advance across their upper surfaces, and gradually, after they have 

 increased to nearly twice their original length, they meet in the median 

 dorsal line. Generally the processes in this case are very slender, 

 and they usually remain in this condition permanently, so that the 

 contact surface is as slight as in the case of the cells of the upper 

 row bridging the gap. Were this condition of affairs to continue the 

 prototroch would be a very feeble structure posteriorly, but it becomes 

 reinforced by two first quartette cells that are derived from the me- 

 dian cell of the cross. The position of these cells may be seen in 

 e, diagram B, or Figs. 45 and 46. Often these latter products 

 divide and the resulting blastomeres arrange themselves in such a way 

 that the prototrochal band is as wide here as in any other part of 

 its course. 



When first formed the trochoblasts apparently differ in no way 

 from the remaining cells of the ectoblast, but as development pro- 

 ceeds and they commence to assume their permanent positions they 

 become characterized by features that are found in no other cells of 

 the embryo. Outwardly these changes manifest themselves by a 



