BIGELOW: EARLY DEVELOPMENT OF LEPAS. 87 



It is evident that this account refers to the processes which I have 

 described in the chapter on maturation of the ovum. They are phenom- 

 ena concerned with the establishment of visible polarity in the egg, 

 and not with the cleavage process, as Groom's account leads us to infer. 

 The surface marking the boundary of yolk and protoplasm, as shown in 

 Groom's Figures 6 and 7 (in this paper Figs. 3 and 18), does not 

 " mark off what will be the first blastomere." Groom evidently mis- 

 took the constriction which I have described in the account of matura- 

 tion (Fig. 3) for the forming cleavage plane ; but I have shown the 

 cleavage plane to be almost perpendicular to this transverse constriction, 

 which merely marks off the yolk-lobe (see Figs. 3 and 18). Groom's 

 misinterpretation explains the cases described by him, in which the 

 cleavage plane appeared transverse and the polar cell apical in position ; 

 see his Figures 6 and 7, which evidently correspond to my Figures 3 

 and 18. Groom has interpreted his Figures 6, 7 and 8 (L. anatifera), 

 and 45, 46 and 47 (L. pectinata) as representing successive stages in the 

 formation of the first cleavage plane. As a matter of fact there inter- 

 vene between the last two stages of each of these series all the stages 

 which are shown in this paper by Figures 4-15. The identification by 

 Groom of the transverse constricting furrow of the maturation period as 

 the forming cleavage furrow has probably led to his erroneous interpre- 

 tation of the position of the polar cell with reference to the first cleavage 

 plane. It was natural that Groom, considering the three figures men- 

 tioned above (Figs. 6, 7, 8) as a continuous series, should expect to find 

 the polar cell at the place of its formation, and should overlook it in the 

 first cleavage furrow. The best of observers could easily have been mis- 

 led, unless an opportunity came for following a single ovum uninterrupt- 

 edly through the maturation and first cleavage stage. The polar cell lies 

 deep in the cleavage furrow, and is easily overlooked in the living ovum, 

 unless one's attention has been attracted to it in prepared ova, where it 

 is clearly shown in the majority of cases. The rare cases observed by 

 Groom of ova in which the polar cell retained its original position in 

 undoubted two-cell stages are explained by my observation that the 

 polar cell sometimes, but very rarely, fails to rotate with the ovum. 

 That the polar cell is not soon lost, as Groom believed, is evident from 

 many of my figures of later stages. In preparations it is as often seen 

 in later stages of cleavage as in the unsegmented ovum. 



Groom's Figure 101 (L. anatifera), showing a longitudinal position 

 of the spindle, is certainly from a section taken in a plane oblique 

 to the chief axis so as to show the spindle in the long axis of the sec- 



