No. I.] POLYCHOERUS CAUDATUS. 163 



Again follows a period of rest and disappearance of all cell 

 outlines. Renewed activity in cleavage is heralded by changes 

 in the pigmentation, and the line of the next cleavage plane 

 is indicated by a distinct line of red granules dividing off the 

 upper portions of A and A^ in the same direction as that of the 

 second and third cleavage planes and presently D and D^ are 

 budded off from A and A^ respectively (Fig. 7). 



After the usual period of rest, the fifth cleavage plane is, as 

 lisual, first indicated by the pigment granules, and then the 

 cells E and E^ are budded from A and A' respectively (Fig. 8), 

 and in a plane parallel to the first but at right angles to all other 

 cleavage planes which have been described. Hence we have a 

 ten-celled stage, eight of which cells have been budded off from 

 the original macromeres A and A\ which exist now as mere 

 remnants. At this stage we have the cells which will form the 

 germinal layers already differentiated, for a little later, by a 

 process which will be described A and A^ pass into the centre 

 of the ovum and form the mesendoderm, while the lineal 

 descendants from B, B\ C, C, D, D\ E and E' form the 

 ectoderm. 



Thus far the segmentation has proceeded with great regu- 

 larity, but from now on the order in which the cells divide 

 varies so much that it is often difficult to determine exactly in 

 which generation certain cells are to be classed. Although the 

 bilateral symmetry is on the whole maintained, it frequently 

 happens that on one end or side, the cells divide more rapidly 

 than elsewhere, so that an ovum presents the appearance of 

 irregularity ; but when this occurs, the cells on the correspond- 

 ing end or side divide in an exactly similar manner shortly 

 afterward, so that the symmetry is soon regained, and the 

 general scheme of segmentation is always the same. In 

 order to make the description of the process more clear a 

 short explanation of the manner in which the cells are lettered 

 may be necessary. The cells A and A^ are the result of the 

 first division of the ovum, and from these B, C, D, E and B\ C, 

 D', E' are given off respectively. In all cases the sign ' indi- 

 cates that the cell is descended from A' and its absence that it 

 is descended from A. Thus we have Ei.r.i descended from A 



