112 BULLETIN OF THE 



Apostolides. No change was detected in the ovum of the latter seven 

 hours after fecundation. On the seventh hour after the contact of ova 

 and sperm, according to Apostolides, the first indications of the segmen- 

 tation of the egg appear in Ophiothrix. Seven hours after the capture 

 of Ophiopholis some of the ova were found in the 4-cell stage, and a 

 little over three hours after the egg was dropped by the female they were 

 found in the 2-cell stage. Is this discrepancy the result of a difference 

 of temperature in the water 1 * 



The second segmentation stage, the 4-cell stage, PL I. figs. 6, 7, is 

 brought about by a formation of a second cleavage plane, 2 cl pi, at 

 right angles to the first. As in the first the two cells of the 2-cell stage 

 are separated by a transparent layer, in the same manner each of the two 

 ceUs of the 4-cell stage are divided by a similar plane. A nucleus is 

 observed in each of the spheres of the 4-cell stage. The division of the 

 2-cell stage into four cells is regular,t and all the cells are of the same 

 size up to the 4-ceU stage. 



Cleavage. 



The first external change in form of the spherical egg in its segmenta- 

 tion is the formation of the first cleavage-plane. A constriction or annular 

 groove, destined to divide the egg into hemispheres, forms about the egg 

 in the same way as in other Echinoderms. It was not observed whether 

 a collar-like J extension of the superficial portion of the yolk sinks into 

 the denser central region, or whether the two cells form in some other 

 way, but in a well-formed 2-cell stage the two cells are separated by a 

 transparent walL The transparent wall is identical with that which 

 covers the denser part of the ovum in its undivided condition. 



The 4-cell stage is formed from the 2-cell stage by a cleavage-plane, 

 2 cl pi, at right angles to the first. As in the 2-cell stage the two cells 

 or blastomeres are separated by a transparent layer, so in the 4-cell stag€ 



* Metschnikotf {Zeit. /. yfiss. Zodl. XLIL, p. 665) has recorded a great differ- 

 ence in the time occupied by the early development of the eggs of the same Echino- 

 derm from two localities. Tliese differences are probably due to temperature. 

 The influence of temperature on the rate of development of the ova of Echino- 

 derms is a subject which would repay an extended investigation. 



t If any irregularity in size exists the difference is very small. 



t It may be supposed tliat the superficial layer following a constriction of 

 the denser part of the ovum, bisects the latter by a centripetal growth- This 

 would correspond with the mode of formation of the 2-cell stage in other Echino- 

 derms. 



