594 WILLIAM PATTEN. 



embryo except by a central umbilicus. It finally, however, 

 becomes completely separated, and in Orchestia sinks into the 

 yolk. 



As Bobretzky has hinted, it is very probable that this saddle- 

 shaped area of cells is homologous with the embryonic mem- 

 branes of insects. If we imagine, for instance, that the ventral 

 plate of Oniscus had extended nearly around the egg and then 

 the embryonic membranes formed just as in insects (without, 

 however, extending completely over the ventral surface of 

 embryo), and if the membranes thus formed should fuse with 

 the vitelline membrane, we should then have an organ which 

 would be like the dorsal organ of Oniscus, and at the same time 

 homologous with the embryonic membranes of insects. If, to 

 carry the comparison still farther, the dorsal plate of cells 

 should separate from the vitelline membrane, and, contracting, 

 sink into the yolk, we should have still further grounds for 

 comparison. In fact these conditions are realised in embryo 

 Amphipods and Isopods, and show that the saddle-shaped plate 

 of cells in Oniscus, &c., undergoes a series of changes, parallel 

 with those of the embryonic membranes and dorsal organ of 

 insects. A careful distinction, however, should be made 

 between the dorsal organ of Insecta, and that of the Phyllopods 

 and Nauplii of Decapods, &c., all of which have been con- 

 sidered by Dohrn as homologous structures. As a matter 

 of fact, however, the only points they have in common is that 

 all are located in the neighbourhood of the dorsum and that 

 very little is known about them. 



It will be observed from the description already given of the 

 germinal band with its gastrular and neural invaginations that 

 a wonderful analogy at least, if not homology, exists between 

 these structures and the corresponding ones in Vertebrates, 

 and especially birds. For instance, in both bird and insect 

 the germinal band has the same slipper-shaped outline. The 

 primitive streak of birds and the gastrula- groove in insects, 

 both of which are homologous structures, begin at the pos- 

 terior end of the germinal band and extend forwards. Like- 

 wise the medullary furrow in both groups arises at the anterior 



