364 PROCEKDIXGS OF THE ACADEMY OF [April, 



mainly concerned in the closure of the blastopore, though the deri^'a- 

 tives of 2a^^-2d^^ also bound the narrowing opening. Planorbis 

 (Holmes) shows a very similar condition, -v^dth the exception that 2d-^ 

 is crowded out. In Fiona all second quartet cells but a few at' [the 

 anterior edge of the blastopore are excluded before the opening closes. 



Orgaxogexy. 

 The Velum. 



In its earlier stages the velum of Fiona is so ill-defined on the upper 

 surface of the developing larva that its study has proved most diffi- 

 cult, and though more time has been spent upon this region than any 

 other portion of the developing organism the results have not been 

 as satisfactory as could be wished. living material would have been 

 of great value, and the lack of it has been a source of much regret. 

 After the breaking up of the cross the whole external surface of the 

 gastrula, and particularly the anterior end, is characterized by cells of 

 small and nearly equal size, among which there appear scarcely any 

 cells whose size would give them prominence, or cell rows or distinctly 

 marked groups. 



In the last stage described under the discussion of the develop- 

 ment of the first quartet the area covered by this series of micromeres 

 represents nearly the whole upper surface of the flattened gastrula (fig. 

 75). The four arms of the cross are split transversely, while in the 

 angles between them lie the four groups of turret cells, each group 

 consisting of four cells of equal size. In axial relation the anterior and 

 posterior arms correspond to the direction of the median plane, while 

 the lateral are respectively right and left. The Avhole first quartet 

 area is completely surrounded and separated from the tliird by deriva- 

 tives of the second. By an increased growth of D quadrant of this 

 series the apical pole and its surrounding area is moved forward in 

 the direction of the blastopore, while at the same time growth of first 

 and second quartet elements in the neighljorhood of the tip of the ante- 

 rior arm of the cross causes that region to become raised, until some- 

 what later the pointed anterior end so characteristic of many Opistho- 

 branch larvas is produced (figs. 78, 79, 96). The visible cause of the 

 evagination of the ectoderm at this point may be found in the direc- 

 tions taken by spindles of the dividing cells which produce it, as in 

 most cases they are radially or diagonally directed toward the point 

 of greatest elevation. At this time the archenteron is roughly trian- 

 gular in outline, the anterior point of the triangle being marked by 



