THE DEVELOPMENT OF THE EARTH-WORM. 215 



however, extend round the whole circumference, but is 

 present on one side only. This furrow, deepening itself, 

 either by the elevation of its borders or because the cells 

 lining its floor force themselves into the lateral elevations, 

 divides the germ nearly completely into two halves, which are 

 joined only by a series of enlarged ectoderm cells. The 

 process of the development of the transverse fissure goes 

 on simultaneously Avith the differentiation of the cells of the 

 liitherto inactive hemisphere. 



To explain better the entire process I will describe 

 figures 4, 6, and 7 of PI. IX. Comparing fig. 4 with fig. 3, 

 which represent two stages very near together, the elonga- 

 tion of the diameter which passes throtagh the poles of 

 the egg will be noticed, transforming it from a sphere to an 

 ovoid ; at one extremity the arrangement of the cells remains 

 exactly as it was in fig. 3, but in the middle the mass of 

 cells is divided by ^ larger fissure, which represents the 

 bottom of the transverse furrow; to the right of this is seen, 

 instead of the simple layer of large cells of fig. 3, two 

 very distinct groups of cells, one peripheral, of nearly cylin- 

 drical cells, and a mass of polygonal cells in the interior, 

 which forms part of the wall of the furrow. 



In fig. 6, Avhich appears a little less complicated in detail 

 only because the plane of the optical section does not pass 

 through the rudiments of the middle layer, which are, how- 

 ever, easily recognisable in the preparation, the division 

 into two halves of like structure may already be distinctly 

 seen, though that on the right hand is still a little behind 

 the other in development, not having the endoderm well 

 defined. Of the two mesoblast cells on this side one only 

 is represented, because the other is hidden by it. In the 

 middle, between the two hemispheres, are to be noticed two 

 large, transversely elongated cells, distinguished by the 

 clearness of their protoplasm, which form a kind of ligament 

 between the two halves. 



Fig. 7 shows the egg distinctly divided into two halves of 

 very similar structure, joined together not very closely by a 

 median cord of large cells containing large nuclei. 



While the transverse furrow deepens the entire egg 

 changes its form and becomes kidney- or bean-shaped, and 

 then the free margins of the groove arch inwards and 

 approach one another in such a way as to narrow con- 

 siderably the entrance. The bottom enlarges in the direction 

 of the extremities and excavates the inside of each of the 

 hemispheres, pushing the cellular layer {e?i) towards the 

 inside. In other words, the endoderm becomes invaginated. 



