ECHINOCYAMUS PUSILLUS. 15 



present themselves as ovals, PI. II, fig. 22. The principal axis of the 

 future larva is indicated by the section line of the two vertical planes. 



Even in this stage of the development a segmentation cavity is 

 present, defined by the four cells. More than once I have seen very 

 delicate connective filaments crossing the cavity from one segment to 

 the other. The cavity in question changes considerably in size accor- 

 ding as the segments themselves vary in form and position. As will 

 be understood from the above statement, the blastomeres first become 

 separated and assume a rounded oval shape, PI. I, fig. 16 — 17, thus 

 leaving between them a rather large cavity. The four blastomeres now 

 begin to press against each other and flatten, the result being that each 

 segment finally resembles a quadrant of a sphere with two plain sur- 

 faces and one convex. In this state of compression of the segments the 

 cavity is of course very insignificant, PL I, fig. 18 — 19. Afterwards they 

 again become gradually separated and consequently the segmentation 

 cavity also increases, PL 11.^ fig. 21 — 22. 



Stadium with eight segments. 



At about one hour and 55 minutes after the fecundation an equa- 

 torial plane of division makes its appearance and divides the four seg- 

 ments into eight of fully equal size. The constriction makes its first 

 appearance at the sides which are turned towards each other. The 

 above mentioned changes in the form and position of the segments 

 take place and meanwhile the central cavity assumes a more or less 

 obvious ellipsoidal form, PL II, fig. 23 — 24. 



Up to this stage of the development, the segmentation of the eggs 

 of Echinocyaraus agrees in the main with that of Echinarachnius and 

 Mellita as described by Fewkes ') and Nachtrieb ^). But with regard to 

 the subsequent stages in the segmentation of the egg of a Clypea- 

 stroid, nothing has hitherto beeji known of any interest. As to Echi- 

 narachnius Fewkes only says: »The segments of the 4-cell stage are, 

 however, not always bisected, and here appears the first indication of 

 an unequal segmentation», and further down, »An egg in the 32-cell 

 stage was found four hours after impregnation». With regard to Mel- 

 lita Nachtrieb confines himself to the following: »After eight blastomeres 



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

 rachnius. p. 125. 



2) Preliminary Notes on the Echinoderms of Beaufort, p. 67. 



