24.e 



s. GOTO ; 



observed by MacBride, not hoinolog-ou.s, and the arrangement of 

 calcareous plates in the two classes must be looked upon as simj^ly a 

 remarkable case of homoplusy. To still maintain the homology of 

 the poles of the same name in the asterid and crinoid would involve 

 an assertion of the homology of the posterior pole of the crinoid hirva, 

 with the anterior pole of thfit of the asterid — an assertion which, for 

 all that we know, would be too arbitrary and too much against flicts. 



The foregoing description has been kept for clearness' sake some- 

 what abstract. Let us now see the process exactly as it occurs in 

 nature. In stage 1j the median portion of the ectoderm lying dorsal 

 to the stomach is considerably thickened (Figs. 21 & 22, PI. XX), the 

 cells being highest in the median line and thence decreasing towards 

 either side. The aboral disc is formed by the extension of this thicken- 

 ing towards tiie right side of the body. In stage C the thickening has 

 extended, posterior to the anus, in many specimens to near the ventral 

 median line (Fig. 17, PI. XX); but this is not always the case, for, as 

 we see from Figs. 19 & 20, PI. XX, which are taken from a specimen 

 in stage C, the thickening is still confined to the dorsal median portion 

 of the body. In stage D, however, the thickening has passed the 

 ventral median line and has extended so far anteriad that its antero- ven- 

 tral margin overhangs the protoproct. In the posterior end of the body 

 also it has encroached on to the rio-ht side. From this stajre onwards, 

 the aboral disc seems to grow, to a large extent though not solely, by a 

 change of the surrounding cells from a flat to a cylindrical form ; for, 

 while mitotic tigures are rarely, if at all, observed in the ectoderm, the 

 prœoral portion, and especially that portion of the body lying between 

 the protostome and the aboral disc, steadily decreases in size, until, 

 passing through stages E and F, it finally reaches stage G, in which 

 the portion adjoining the mouth has been already incorporated into 

 the oral disc ; and the prœoral lobe hangs down as a mere appendage 



