1007.] NATURAL SCIEXCES OF I'll 1 1, A DKI.I'III A. 337 



ectonieres these cells become very miuierous in FuUfur, as shown in 

 figs. 17, IS, ct scq., whereas in Crcpulula the number remains relatively 

 i^mall. It is evident that the same cell stages in the two no longer 

 rejM-esent similar stages in differentiation; for example, the shell gland 

 is one of the first organs to appear, and is first visible in Crepidula 

 when tliere are about 250 cells present (Conklin, 1897, fig. 74); while 

 it fii-st becomes visible in Fuhjur when there are about 1,000 cells 

 (fig. 23). The period of the closure of the blastopore is very different 

 in the two; in (JrcjiiduUi this occurs before any organs are visible (fig. 

 65), in Fulgur only after almost all the larval and many of the defini- 

 tive organs are present (fig. 31). 



Other striking differences between Fulgur and Crepidula are found 

 in the form and character of the different organs and in the time at 

 which they appear, but most of all in the method of formation of 

 these organs and in their relation to the yolk. 



1. OVERGROWTH OF THE YOTJv AND COXCHESCEXCE OF THE EMimVO. 



Owing to the fact that the entomeres are so large in Fulgur, organs 

 begin to differentiate long before the closure of the blastopore, indeed 

 while the blastoderm is still a small cap at the animal pole of the egg 

 (figs. 23-26). The organ bases are here spread out as in a mercator's 

 chart, whereas in Crepidula the organ bases do not appear until the 

 blastoderm has overgrown the yolk and they occur over the sphere, so 

 that only a few of them can be seen from a single point of view. 



In the earlier stages of Fulgur the organ bases occur in the anterior 

 half as well as in the posterior portion of the blastoderm (figs. 21-23) ; 

 later by the very rapid growth of the median anterior portion of the 

 blastotlerm these organ bases are displaced laterally and posteriorly 

 until they come to form a kind of germ ring, or crescent, along the 

 posterior margin of the blastoderm (figs. 24-28). This germ ring 

 contains the bases of all the future organs, whereas the rest of the 

 blastoderm forms a kind of yolk sac. 



The manner of the overgrowth is represented diagrammatically in 

 the accompanying text figure. The outlines of the blastoderm in 

 successive stages of the overgrowth are indicated by the numerals 1-S. 

 By superimposing on the same figure these different stages of the 

 overgrowth it is possilile to see at a glance the character and 

 relati\'e amount of the movement of the different organ bases. 

 Thus in 1, the center of the blastoderm is approximately the center 

 of the animal i)ole; in front of this lie the cerebral ganglia (c5) 



