176 EDWIN G. CONKLIN 



later stages, the number of ectoderm cells being greater in the 

 larger eggs than in the smaller ones. Up to the 52-cell stage the 

 number of cells is the same in the eggs of all the species examined; 

 at this stage one additional ectoderm cell appears in the posterior 

 arm of the 'cross' in C. adunca which does not appear until later 

 in the other species (the additional cell is the one shaded by trans- 

 verse lines in fig. 10). At the 82-cell stage four such additional 

 cells are present in C. adunca, two in the posterior arm of the 

 cross and two in the posterior 'turret' cells, all the other cells 

 being the same in all the species (figs. 11, 12). .These additional 

 cells of the posterior arm of the cross and of the posterior 

 turret cells are all similar histologically as well as in 'prospective 

 significance' with the cells from which they were derived. They 

 all become the large ciUated cells of the 'posterior cell plate' 

 (Conklin, '97, p. 109), and ultimately give rise to the large cells 

 which form the head vesicle of the larva. In the later stages of 

 cleavage many additional ectoderm cells appear in the larger 

 eggs which are not present in the smaller ones. Most of these 

 additional cells appear in the primordia of organs after these have 

 been established, and consequently represent only an increase in 

 cells of a given kind. These primordia form chiefly on the oral 

 side of the egg, whereas the greater part of the aboral hemisphere 

 is covered by the large cells of the posterior cell plate. As a cpn- 

 sequence the number of cells visible from the aboral pole at the 

 period of the closure of the blastopore does not differ greatly in 

 number in the different species; whereas the cells visible from the 

 oral pole differ greatly in number in the different species, there 

 being more than three times as many in C. adunca as in C. plana 

 at the time when the blastopore closes, and in later stages this 

 disporportion becomes much greater. 



Table 5 gives the diameter of the egg and of the ectodermal 

 plate in different species of Crepidula, at the time when the ecto- 

 meres are first separated from the macromeres; also the approxi- 

 mate number of ectoderm cells, visible from the oral and the 

 aboral poles at the time of the closure of the blastopore, in the 

 different species. 



