ECHINOCYAMUS PÜSILLUS. 19 



cited he speaks of a considerable difference in size between the seg- 

 mentation spheres in the fourth stadium of cleavage and some years 

 later Fleischman *) states the same with regard to Echinocardium cor- 

 datum. Thus, it may be a rule among all the Echinoids, that a real 

 difference in the size of the segmentation spheres first occurs in the 

 fourth cleavage stadium ^). 



IV. 

 The Blastula stage. 



Shortly after the end of the seventh or eighth hour after the fecun- 

 dation has taken place, the blastomeres have arranged themselves so 

 as to form a spherical Blastula or Blastosphere, PL II, fig. 31. In other 

 cases this stage of the development is accomplished more or less rapidly 

 according as the temperature is higher or lower. There is no solid 

 morula stage. The young Blastula reaches a diameter of about Ü,i mm., 

 and its segmentation cavity, which is filled up with a clear homoge- 

 neous substance, measures about 0,06 mm. This homogeneous substance, 

 Hensen's »Gallertkern», is apparently a product of the activity of the 

 blastomeres from the time when the first trace of a segmentation cavity 

 becomes visible, and is probably, at least until the blastula stage is 

 reached, of the same plasmatical nature as the homogeneous plasma 

 which at the moment of fertilization arises round the yolk and removes 

 the vitelline membrane. As has been mentioned above, this plasma also 

 is the result of the activity of the undivided egg-cell at the moment 

 of fertilization. 



The cells which constitute the Blastoderm, have lost their round 

 shape from mutual compression and obtained a slightly cylindrical form : 

 viewed from the surface, each cell has a polygonal outline, while in 



1) 'Die EntwickluDg des Eies von Echinocardium cordatum. — Zeitschr. f. wiss. 

 Zool. XLVI. 2. 1888. p. 136—137. 



2) According to Brooks — Handbook of invertebrate Zoology, Boston 1882 — 

 all the segments in Arbacia punctulata seem to be equal, as may be also understood 

 from his figures, drawn by Gaeman. Unfortunately, I have not been able to see the 

 original »Notes» on Arbacia punctulata by Colton and Garman. Further investiga- 

 tions will prove whether Brooks is right in his statements. 



