20 T. H. MORGAN. 



At the next stage, shown by Fig. 24, the arching over is com- 

 pleted and fusion has taken place, so that there is a cavity in 

 each ganglion. The cavity does not run through the whole 

 length of each ganglion, but lies only in its middle portion. 

 There is no communication between the cavities of different 

 ganglia. In this figure each ganglion has increased in size, and 

 the cells have become more numerous, and, further, the neighbor- 

 ing ganglia are connected by a cross commissure of fibers. 

 Further, the nuclei of the outer cells, those lining the cavity, are 

 now more like the nuclei of the ganglion cells. Later stages 

 show that the central cavity of each ganglion disappears, 

 although they seem to persist for quite a long time after they 

 have been enclosed by the ganglia. That this structure may 

 have represented an organ in some ancestral form I believe 

 possible, first, because it is distinctly marked, differentiated, 

 before any invagination takes place; secondly, because their 

 cells are histologically distinct from the surrounding ectoblast 

 and from the cells of the ganglion beneath, and lastly, from the 

 arrangement of the cells, which suggests a sense organ comparable 

 to the simple ectodermal organs of many animals. It also seems 

 probable that we have exactly similar structures in the Pantopod- 

 larva of the other Pycnogonids. 



Returning to stage III we have a cross-section of the body 

 drawn in Fig. 25. The section passes through the middle of the 

 body in the plane of a pair of walking legs. The large mass of 

 yolk is still seen filling the upper part of the embryo. Over the 

 ventral surface of the yolk the entoblast cells form a continuous 

 covering and are indicated by JV in the figure. At the base of 

 the appendage the entoblast sends out a prolongation which is 

 filled with yolk, while the entoblast cells are here higher and 

 more closely packed than elsewhere. Into each of the six 

 walking legs is a similar protrusion of entoblast from the mid- 

 gut. Beyond these diverticula there is in each leg a definitely 

 marked cavity in the mesoblast. In Fig. 25 the surrounding 

 mesoblast is shown by M. As the prolongations of the mid-gut 

 push into the legs these cavities cannot any longer be made out, 

 and either become lost or are too complicated to follow out. 

 Besides these cavities, which would seem to be body-cavities, we 

 shall come to others later in development which seem to be 



