No. I.] THE EMBRYOLOGY OF CREPIDULA. 95 



wholly different axial relations ; besides, the arms are never more 

 than two cells long, though they may become three cells broad. 

 The real cross, i.e., the structure homologous with the cross in 

 Neritina and Crepidula, is plainly present in his figures, and 

 its close resemblance to the same structure described by Bloch- 

 mann and myself is all the more striking, since apparently 

 Heymons did not recognize its presence. 



It is composed of cells of exactly the same derivation and of 

 relatively the same size and position. Thus the terminal cells are, 

 using my nomenclature, 2a''-2d'', and they arise by laeotropic 

 division ; the basal cells are la'^^-rd'-^ and they arise by dexio- 

 tropic cleavage. The two cells of each arm, and especially their 

 nuclei, lie in line with one of the apical cells, and a line drawn 

 through the nuclei of these three cells forms a curved radius, the 

 four radii being dexiotropic. Heymons especially says of the 

 terminal cells (of course he does not use this designation) : " Es 

 sind dies die kleinsten Ektodermzellen welche bisher gebildet 

 wurden." To all of these facts I have already called attention 

 in Crepidula, and in general they seem to be true of Neritina. 



In the time of its formation the cross in Umbrella shows 

 some interesting differences from the cross in Neritina and 

 Crepidula, e.g., the terminal cells are first formed and the basal 

 cells are not formed until a considerably later period. The 

 arms are more curved in a dexiotropic direction than in either 

 of the other gasteropods, and the whole cross is less clearly 

 marked off from the surrounding cells. But most important of 

 all the differences is the fact that the first division of the basal 

 cells ts laeotropic in Umbrella, Diagram 12, d, while it is hivari- 

 ably dexiotropic in Crepidula and Neritijia. Upon this difference 

 the future recognizability of the cross in the last-mentioned 

 cases depends. If these basal cells should divide in Neritina 

 and Crepidula, as they do in Umbrella, there would be no cross 

 after the stage in which there are two cells in each arm. The 

 existence of the cross in the later stages depends upon the 

 direction of this one division. It is therefore all the more 

 interesting to note that this division in Umbrella follows the 

 usual rule of alternation of direction, whereas in Neritina and 

 Crepidula it violates that rule. 



