300 MEAD. [Vol. XIII. 



pole. A line connecting the two poles is the &%^ axis. We can 

 distinguish neither the prospective sagittal plane, nor the right 

 and left sides of the trochophore, until the first cleavage spindle 

 appears. This spindle indicates the direction and position of 

 the first furrow, and all the other furrows follow in perfectly 

 regular and constant sequence. At the 4-cell stage the pros- 

 pective sagittal plane cuts B and D as in most annelids, — Clep- 

 sine, Rhynchelmis, Clymenella, Aricia, Polyjiinia, etc.; and in 

 Unio and Discocoelis. The cells A, B, C, and D are the 

 anlagen respectively of the left, ventral, right, and dorsal quad- 

 rants of the trochophore. Therefore, the second cleavage furrow 

 does not coincide with the sagittal plane of the embryo. 



When the four cells divide into eight, the upper quartette 

 alternates with the lower so that the second cleavage furrow 

 is twisted and its course more nearly coincides with the sagittal 

 plane on the upper than on the lower hemisphere. From this 

 time the number of cells increases in geometrical progression 

 up to sixty-four, without effecting any considerable redistribu- 

 tion of material. The sixty-four cells are disposed in sixteen 

 alternating quartettes, and the tg^ is capable of very precise 

 orientation with respect to the sagittal plane, anterior and pos- 

 terior ends, ventral and dorsal, right and left sides. 



Now, if the second furrow is followed around the whole &%%, 

 its course is found to be an irregular zigzag, but its general 

 direction is at a considerable angle to the sagittal plane. The 

 embryonic material is segregated in such a way that the meso- 

 derm is contained in one cell, the entoderm in seven; of the 

 fifty-six remaining ectoderm cells, sixteen are already function- 

 ing as prototroch and four constitute the somatic plate. The 

 prototrochal cells naturally divide the embryo into an umbrellar 

 and a subumbrellar hemisphere. On the latter the principal 

 shifting of areas, which characterizes the regional metamor- 

 phosis, takes place. The regions which figure as units during 

 the transformation are the somatic plate, the mesoderm and 

 entoderm, and the remaining subumbrellar ectoderm. 



The main features of the metamorphosis are as follows: the 

 mesoderm and entoderm sink into the segmentation cavity, and 

 the somatic plate thins out and extends over the large area thus 



