ECHINOCYAMUS PUSILLUS. 13 



slightly unequal; as to the edentate Echinoids Fleischman *) points out 

 that they are of unequal size, but judging from the figures which he 

 gives the difference must even here be very inconspicuous if it exist at 

 all. Considering this, it appears credible that the very first stages of 

 the development as well as the whole process of segmentation, which 

 will be understood from the following, are displayed in about the same 

 manner in the diff"erent groups of Echinoids. 



Stadium with four segments. 



The changes which take place not only in the mutual position 

 of the two blastomeres but even in their external shape are rather re- 

 markable and, as it seems to me, have not been sufficiently regarded. 



At first the two segments, as seen from the side, are oval and 

 slightly flattened against each other, PL I, fig. 10. 



Then they acquire a more spherical shape so that the area of 

 contact becomes very inconsiderable, PL I, fig. 11. 



After this, they press gradually towards each other, with the 

 result that the area of contact becomes large and flat. 



Finally the segmentation spheres almost assume the shape of 

 hemispheres, so much are they pressed and flattened against each other. 

 When the large flat areas are most closely united, the nucleus has be- 

 come most distinct and its contour well defined, PL /, fig. 12. 



Subsequently they begin again to separate, which process is in- 

 troduced by the nuclei ceasing to be visible, PL I, fig. 13., and begin- 

 ning to repeat the well known important changes. As this separation 

 proceeds, each segment begins to present the two star-shaped figures 

 formed by radial striae, PL I, fig. 14.^ which become very distinct when 

 the two segments have fully separated so as to present themselves as 

 two oval spheres placed side by side. 



From the above mentioned changes, which take place during 

 the transformation of the 2-cell stage into the 4-cell stage and which 

 in each subsequent division are to be repeated, it is evident not only 

 that the clear protoplasm surrounding the nucleus in each segment and 

 the nucleus itself are in restless activity during the processes of seg- 

 mentation, but also that the remaining part of the segments, which is 



1) Die Entmckking des Eies von Echinocardium cordatum. — Zeitschrift f. 

 wiss. Zoologie. Bd. 46. 2. 1888. p. 134—135. 



