The Development of IscbnochitOD. 591 



alone, but is a more profound movement that extends itself into other 

 sets of cells. 



First, regarding the method by which la^-^-^ '-^ and f d- ^ '-^ come 

 in contact. In their early stages these cells are cuboidal in shape 

 (Figs. 26, 27) but shortly after the division to form the tip cells they 

 will be seen to become somewhat elongated on the side next the tip 

 cells and their angles on that side will become more and more acute 

 (compare a and b, diagram B). In other words they become wedge- 

 shaped with the pointed ends tending to come in contact with each 

 other. This movement and change of shape becomes more marked, 

 and they seemingly pry apart the cells of the first quartette (some 

 median cells of the cross), and the tip cells lying below them. 

 Generally this change of shape affects the entire cell, but frequently 

 the wedge-like portion includes but half the cell, the remaining half 

 appearing as a more or less globular mass. The appearance may be 

 compared to an amoeba with one pseudopodium which pushing out 

 ultimately comes in contact with a like process from a neighboring 

 cell. Some specimens show the processes to be extremely slender, 

 sometimes requiring careful focussing to observe them, but after the 

 two cells come in contact they gradually assume a rectangular form 

 which they retain throughout development. By this means spaces 

 nearly or quite as wide as the upper border of the tip cells are 

 bridged in three quadrants. The upper row of the velum is now 

 complete, and consists throughout of first quartette trochoblasts 

 (diagram B, c). 



In the meantime changes have occurred which produce a continuous 

 lower row in which /«^---^ and id^^--^ are in contact with the tip 

 cells. In the earlier stages these latter cells are isolated from any 

 of the lower row, but two series of movements bring about the final 

 continuity. The most important are the changes undergone by the 

 tip cells. As a general thing they are more or less triangular with 

 their bases in contact, and their apices directed outward toward the 

 cells of the lower row. They always remain connected in this manner, 

 but as the embryo develops the bases become smaller and the altitude 

 greater, by which means the apices lie much nearer the cells la'^-'^-i 

 and Id^-'^--. This will be made more clear by a comparison of 

 a and b, diagram B. At the same time a movement generally occurs 

 in the lower row of primary trochoblasts by which the cells become 

 more or less rectangular. Frequently the extremities of la----^ and 

 2d 2. 2. 2 lying next to the secondary trochoblasts become sharply 



