22 Hjalmar Théel, 



If we examine the Blastula stage of other Echinoids, we see 

 clearly that they deviate considerably in external shape from that known 

 in Echinocyamus. Thus, according to Nachtrieb') and Fewkes ^) the Bla- 

 stula in Mellita and Echinarachnius is more or less distinctly spherical. 

 According to Selenka ^) the Blastulse of Echinus miliaris, Toxopneustes 

 brevispinosus, Strongylocentrotus lividus and Arbacia pustulosa have, 

 at least to judge from the figures of the author, an almost spherical shape 

 and are apparently devoid of any cell-differentiation at the animal pole. 

 In a more recent work the same investigator *) gives an account of the 

 segmentation phenomena in Strongylocentrotus lividus, Sphserechinus gra- 

 nularis and Echinus microtuberculatus; here also one arrives at the fact 

 that the Blastula is spherical or slightly oval and furthermore that in 

 the last mentioned form a differentiated cell-area is present at the ani- 

 mal pole. In the explanation of the plates Selenka names this area 

 »Scheitelzone». According to A. Agassiz ^) the Blastula of Strongylocen- 

 trotus dröbachiensis is spherical and, as far as I can see, devoid of 

 any cell-differentiation at the animal pole. Prouho **) describes the Bla- 

 stula of Dorocidaris papillata as spherical and provided with longer cilia 

 at the animal pole, but judging from his figure the cells themselves are 

 not extended in length. Finally according to Debrès ^) the Blastula of 

 Echinus esculentus also becomes spherical, but he does not say any- 

 thing about the animal pole. 



The above cited authors lead one necessarily to the opinion 

 that the Blastula in the regular Echinoids presents itself as a rule in a 

 spherical or slightly oval shape. With regard to the differentiation of 

 the cells at the animal pole, and the formation of a thickened apical 



1) preliminary Notes on the Echinoderms of Beaufort. 1885. p. 66. 



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

 narachnius. 1886, p. 126. 



3) Keimblätter und Organanlage der Echiniden. — Zeitschr. f. Wiss. Zool. 

 Bd. XXXm. 1879. 



4) Die Keimblätter der Echinodermen. Wiesbaden 1883. p. 44. plf?. V — VII. 



5) Revision of the Echini. IV. — Ulustr. Catal. of the Mus. of Comp. Zool. 

 N:o Vn. Cambridge 1874. p. 710. 



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

 Méditerranée. — Arch. d. Zool. expér. Sér. U. t. 15. Paris 1887. p. 343. pi. XXIII. 



7) Observations sur le Mécanisme et les phénomènes 'qui accompagnent la 

 formation de l'embryon chez l'Oursin comestible. — Ann. d. Se. nat. Sér. III. Zool. t. 8. 

 Paris 1847. p. 91. pi. 5. 



