PHOTOGENIC ORGANS OF LAMPYRIDS 175 



neural groove, G, is quite deep, or nearly closed. The lower 

 layer, M, has proliferated so as to constitute almost the entire 

 dorsal portion of the embryo. Development has, as usual, pro- 

 gressed more rapidly at the anterior than at the posterior end 

 (compare figs. 47 and 48). Figure 48, a cross section through 

 the thoracic region, shows several large cells, NB, lodged in the 

 ectoderm, on either side of the median line. These are the 

 neuroblasts which are just beginning to give rise to ganglionic 

 cells. They are not nearly so symmetrically arranged as are 

 those of the orthopteron Xiphidium, studied by Wheeler ('90). 

 A dividing cell, which protrudes from the ventral side of the 

 embryo, strongly suggests the beginning of an appendage (X, 

 fig. 48). By a process of splitting, the mesoderm, M, has given 

 rise to the primitive coelom, IC. Its ventral are thicker than 

 its dorsal walls. More posteriorly (fig. 47), neither the neuro- 

 blasts nor the primitive coelom are as yet discernible. 



A stage intermediate between those of figures 39 and 41, is 

 shown in cross section in figure 49 (abdomen). The sides of the 

 body are further reflexed, the ganglia are marked swellings and 

 contain a large number of ganglion cells which are lodged in a 

 paler fibrillar matrix. The legs, LG, are not yet invaded to their 

 extremities by mesodermal elements. The primitive coelom, 

 IC, is no longer slit-like and appears in horizontal section as a 

 pair of lateral sacs in each segment. Such sacs were found in 

 the mandibular and all the following somites. The permanent 

 body cavity or schizocoele (C, figs. 49, 50 and 51), is formed 

 partly by the withdrawal of the inner walls of the coelomic 

 sacs from the yolk-mass, and partly by the breaking down of 

 the walls of these sacs. In the schizocoele are a few large free 

 cells, with vacuolated protoplasm and large nuclei. These are 

 probably haemocytes, H, which seem to have become separated 

 from the inner, upper wall of the coelomic sacs, the splanchnic 

 mesoderm, which early breaks down at this point. Just dorsal 

 to the ganglionic swelling are several large pericardial cells, PC, 

 which stain more deeply yellow. Like the haemocytes, they 

 appear to be derived from the splanchnic mesoderm. 



Figure 52 is a sagittal section of a more advanced embryo, 

 such as is illustrated in figure 41. The yolk-mass is being 



