ECHINOCYAMUS PUSILLUS. 35 



In Ecbinocyamus the deposit which is originated by the agency 

 of these cells, takes at first the form of a tetrahedron measuring up to 

 0,004 mm., while the cells themselves have a diameter of 0,oi mm., PL IV, 

 fig. 74 — 75. Semon *) was the first to explain the tetrahedron as the 

 primordial form of the deposits in other Echinoderms. The minute 

 tetrahedron grows rapidly to a small star with three very short arms, 

 PI. IV, fig. 69, acquiring a shape almost completely corresponding to 

 the interspace between three close-lying calciferous cells. Thus one is 

 almost tempted to think that there exists a certain relation between the 

 form of the deposit and the interspace in question. The calciferous cells 

 having placed themselves close to the ectoderm, the deposit becomes 

 pressed between them and the latter, PL /F, fig. 77 . If it be so, that 

 this interspace decides the outline of the star, one would expect always 

 to find it in its early developmental stage placed just between the 

 calciferous cells. This seems, however, rarely to be the case. Mostly I 

 have noticed the star situated by the interspace with the arms upon the 

 three cells, PL IV, fig. 69 — 7Ö, and not between them, and sometimes I 

 have seen the star itself somewhat displaced, PL /F, fig. 78. Not- 

 withstanding this, I cannot free myself from the thought that the cells 

 mechanically exercise influence on the outline of the tetrahedron and the 

 star in the earliest stages of the development. 



Before I go further in the description of my own investigations, 

 I may be allowed to state in a few words the two views which have 

 been suggested with regard to the origin of the calcareous deposits in 

 the' Echinoderms ^). 



Selenka •'), who first ventured to treat this question, shows that 

 in the interior of two bilaterally symmetrical cells a minute calca- 

 reous body becomes deposited, that this grows into a regular three- 

 armed figure, and that the cell itself moves towards one of the arms, 

 still persists in depositing calcareous salts (and organic substances) and 



1) Beiträge zur Naturgeschichte der Synaptiden des Mittelmeers. I. Mittheil. 

 Zool. Station zu Neapel. Bd. VII. 2. Berlin 1887. p. 293. 



2) Besides these two views some others have been suggested with regard 

 to the origin of the calcareous deposits in the Holothurioidea, but as they are not 

 founded on embryological investigations I only refer to them: Sejipee (Holothurien 

 — Reisen im Archipel der Philippinen. Leipzig 1868'. p. 29 — 32), ThÉel (Challenger 

 Report — the Holothurioidea. I. 1882. p. 125.) and Hérouaed (Recherches sur les 

 Holothuries des côtes de France. Thèses. Paris 1890. p. 17—29.). 



3) Keimblätter und Organanlage der Echiniden. 1879. p. 46. 



