22 T. H. MORGAN. 



schizocoels. The uppermost of these, lying in the middle line, is 

 the heart, H. At the sides of the mid-gut appear two well- 

 marked cavities, and a third between the nerve ganglion and 

 the outer wall. Also below these are two or more spaces, but 

 these latter, and perhaps some of the others, are artefacts. The 

 mesenteron is covered on its outer side by a distinct layer of 

 mesoblast cells. Cross-section of some of the legs lying below 

 the body is also shown in the figure. 



Between stage VI and stage VII- VIII there is a considerable 

 gap. During this period the embryo has lengthened and the 

 fourth pair of walking legs has appeared. The rudimentary 

 abdomen has been pushed up dorsally and the proctodeum 

 invaginated until at stage VII it has opened into the mesen- 

 teron. The eyes are now seen, but at stage VI they are seen in 

 sections as four thickenings of the epiblasts above the brain. In 

 stage VII- VIII we find by sections that the yolk has almost 

 completely disappeared from the digestive tract. Cross-sections 

 also show the large nerve ganglia, the mid-gut with its diverti- 

 cula, and on the dorsal part of the mid-gut the tubular heart 

 running from the first walking leg to the third. 



Fig. 29 is from a cross-section through the posterior part of 

 the embryo and cuts the mid-gut at its juncture with the procto- 

 deum. Below the digestive tract D is a pair of small ganglia, 

 V 1 , which are the ganglia of the rudimentary abdomen. On 

 account of the shifting of the abdomen to the dorsal side these 

 ganglia have been carried above the last ventral ganglia, which 

 are shown at V 6 . But this is not all, for this sixth pair of 

 ganglia has likewise been afiected by the shifting of the abdomen 

 and in turn lies above and back of the fifth pair of ganglia. In 

 the sixth pair of ganglia, as shown in the figure, we have evident 

 traces of the ventral organs. In the upper part of the section, 

 and on each side of the middle line, are the diverticula of the 

 fourth pair of walking legs. A few scattered mesoderm cells 

 appear in the cavity of the body. The embryo must leave the 

 male at about this time, for I have never found older embryos 

 attached and only exceptionally ones so old as these last figured. 



Comparisons. 

 Comparing the embryology of the two types represented by 

 Phoxichilidium and Pallene, we find that most of the changes of 



