No. I.] THE EMBRYOLOGY OF CREPIDULA. 169 



seem to indicate that the posterior growth of the embryo was 

 not greater than the anterior or lateral growth, but such is not 

 the case, as is strikingly shown by an examination of the aboral 

 side of the embryo, Figs. 64, 74. It is here seen that while 

 the greater growth of ectoblast at the posterior side of the o.^^ 

 has not changed the position of the blastopore, it has shoved 

 the whole of the ectoblast on the aboral surface forward through 

 an angle of about 90°. In this shifting of the ectoblast the 

 entoblast cells take no part, as is clearly shown by the position 

 of the four macromeres and the polar furrow both before and 

 after this movement of the micromeres. Figs. 64, 74. At this 

 stage the polar furrow marks the middle of the dorsal area, the 

 blastopore lies at the middle of the ventral area, and the apical 

 cells which form the centre of the cross lie on the anterior side 

 of the egg about 90° from either of these points. This forward 

 shifting of the apical pole is due to two causes : first, to the 

 enormous enlargement of the cells of the posterior cell plate 

 (Figs. 74, 'j'j), and second, to the rapid multiplication of cells 

 in the region of the shell gland. 



In later stages by the enlargement of the shell gland and the 

 multiplication of cells just ventral to it (the growing-point) the 

 ectoblast of the ventral surface is shifted forward, so that 

 the mouth is carried anteriorly. Fig. 'j6, until it comes to lie 

 near the apex, as shown in Fig. 78. At the same time the 

 embryo increases greatly in length, chiefly by growth at the 

 posterior end, and marks of the final asymmetry appear. 

 The posterior end of the embryo is marked during all this 

 time by the anal cells and by the distal end of the intestine. 

 Figs. 76-^8 ; the antero-posterior axis is at first a straight line 

 connecting the apical and the anal cells. Fig. yy. 



This change of axis is undoubtedly the same phenomenon 

 which has been described by Fol ('75 and '76) in the case of 

 pteropods and heteropods, by Blochmann ('81) in Neritina, by 

 Heymons ('93) in Umbrella, and by Lillie ('95) in Unio. The last- 

 named observer has given such an excellent summary of all these 

 cases except that of Neritina that I need not discuss them here. 



In one point Blochmann' s account of this shifting of the 

 axes in Neritina differs from my observations on Crepidula. 



