12 Hjalmar Théel, 



at the same time the clear plasma is transported to the poles of the 

 nucleus, thus giving rise to a figure resembling a pestle PL I, fig. 6 — 7. 

 After this, the polar stars become very distinct, PI 7, fig. 8, each occu- 

 pying a half of the yolk. Now the metamorphosis of the nucleus enters 

 on a new phase; the nucleus totally passes out of view, the two accu- 

 mulations of clear plasma, which constitute the centre of each star, 

 gradually lose their connection, and the radii of the stars extend in all 

 directions round each clear plasma. 



At 40 or 50 minutes after the impregnation the radii of the stars 

 become most obvious and now the moment of cleavage is present, PL 

 I, fig. 9 — 10, the yolk beginning immediately to divide into two equal 

 segments or spheres, the act of segmentation having been induced 

 by a slight constriction or furrow at the animal pole of the ovum. 

 The first cleavage plane is a vertical one. As soon as the two 

 segments have become separated from each other, the protoplasmic 

 radiation into them disappears. The new nucleus in each segment 

 remains indistinct for some moments, but shortly presents itself as a 

 well defined round body. 



According to Fewkes *) the two first segments in Echinarachnius 

 are also of equal size, and with regard to Mellita Nachtrieb ^) says : 

 »the first two planes of segmentation (meridional) and the third, which 

 is equatorial, divides the egg into eight blastomeres of equal size — 

 occasionally the four at one pole are a trifle larger than the four at 

 the opposite pole». 



If we consider the phenomena of the first segmentation in the 

 dentigerous regular Echinoids and the edentate ones, it will be seen 

 that in the former, according to Selenka ^), Prouho *) and others the 

 first spheres of segmentation are equal in all respects or but very 



" ^ 



1) Preliminary Observations on the Development of Ophiopholis and Echi- 

 narachnius. — Bull. Mus. Comp, zoology at Harvard College. Vol. XH. N:o 4. Cam- 

 bridge 1886. p. 123. 



2) Preliminary Notes on the Echinoderms of Beaufort. — Johns Hopkins 

 University Circulars. Vol. TV. N:o 38. Baltimore 1885. p. 67. 



3) Die Keimblätter der Echinodermeu. Studien über Entwickelungsgeschichte 

 der Thiere. H. Wiesbaden 1883. p. 34. 



4) Recherches sur le Dorocidaris papillata et quelques autres Echinides de la 

 Méditerranée. — Archives de Zool. expérimentale. Sér. H. T. 15. Pai-is 1887. N:o 3. 

 p. 348. pi. XXni, fig. 3. 



