16 0. 0. Whitman 



spìcuous ; bat must he attributed to the mere accident of bis not baving 

 made analyses in favorable cases. Stili, botb bis descriptions and bis 

 figures prove tbat be bad botb species under bis eye. Tbat bis Fig. 1, 

 PI. 2, was taken from an example of D. moscbatum , is unmistakably 

 sbown by the contour of the head and the presenee of two granular 

 verruciform eells. The enneamerous calotte analyzed in bis figures 4 

 and 5, PI. 1, is totally unlike tbat borne by tbis species, and so conspic- 

 uously so tbat it is difficult to find an ingenuous apology for any con- 

 fusion in regard to tbem. 



A more detailed descriptiou may uow be given of the dififerent 

 views of tbis calotte, seen in Plates 1 and 5. Long before the embryo 

 is ready to abandon its parent. the eight uearly equal polar cells are 

 easy to recognize (fig. 92 PI. 5). In the fully formed embryo, seen in 

 the lower part of fig. 1, PI. 1, ali the fundamental features of the calotte 

 are clearly defined. At tbis time its diameter never exceeds the widtb 

 of the body by more than a trifie , and is generally a little less than its 

 axis. Its outline merges bebind in tbat of the body, and is symm.etric- 

 ally rouuded in front. It sekloni betrays the tendency to obliquity 

 manifested in later stages. There is no manifest Constant iuequality in 

 size between the cells of the anterior and posterior set, or between those 

 of opposite sides. The metapolars are cubical, with concave inner and 

 convex outer faces ; the propolars are triangulär-pyramidal , with one 

 eonvex outer face and three piane faces by which tbey are united to 

 each otber and to the metapolars. The rounded vertices of the propo- 

 lars form the anterior end of the embryo : and the middle point, forming 

 the termination of their common line of junction, or axial line, and mark- 

 ed by the intersection of the two dividing planes, may be called the pole 

 of the calotte. The rounded anterior end of the axial cell penetrates a 

 little the basai piane of the four propolars , forming there a small con- 

 cavity, destined to increase until these cells assume together a watch- 

 glass form. 



In fig. 93, PI. 5, is seen a calotte of a very young individuai (.19 mm 

 in lengtb), found free, in wbicb the diameter exceeds the axis, and the 

 propolars are decidedly smaller than the metapolars. Others of a cor- 

 responding age were obtained at the same time in which these dififer- 

 ences were less marked, or eutirely absent. The dorso-ventral sym- 

 metry is quite as perfect as the bilateral. The parapolars , meeting in 

 the median line of the dorsal and the ventral side, form tbus a complete 

 cervical collar, precisely as in the younger stage of fig. 92, wbere their 

 lateral angles are much sborter and more obtusely pointed. Of the two 



