DEVELOPMENT OF PHEYGANIDS. 587 



yolk. A longitudinal section of this invagination is to be seen 

 in Pl.XXXVIc, fig. 39. The whole dorsal organ now sinks into 

 the yolk, the nuclei become less distinct, and by the confluence 

 of its sides the invagination disappears (fig. 40) . Still later no 

 trace of the nuclei can be detected ; they have gradually faded, 

 and finally disappeared altogether from the protoplasmic mass 

 without shifting their position. The protoplasmic matrix 

 in which the nuclei were embedded does not break up into 

 fragments, but disappears en masse some time after the 

 nuclei. 



After the rupture of the membranes, and during the changes 

 which take place in the dorsal organ, the abdomen of the 

 embryo becomes folded more and more towards the ventral 

 side, until the curvature of the embryo becomes completely 

 reversed (compare PI. XXXVIa, figs. 12 and 16). 



After the dorsal organ has passed into the yolk, the lips of 

 the uprising mesoderm, which on either side unite the splanchnic 

 and somatic layers, gradually envelop the yolk, and finally meet 

 each other along the middle line of the dorsum. By the fusion 

 of these two mesodermic lips a solid cord of cells is formed 

 occupying the median longitudinal line of the back indicating 

 the first foundation of the heart. A cross section of the heart 

 at this period is represented in PI. XXXVIc, fig. 32, from which 

 it will be observed that the cells have fused into a common 

 mass of protoplasm in which the nuclei are irregularly 

 arranged. 



Transverse sections through the embryo just before hatching 

 show that this cord of cells has become hollow ; the cavity 

 thus formed is the lumen of the dorsal blood-vessel.^ I am 

 unable to say whether any of the nuclei of this cord are 

 isolated and fall into the lumen during the histological diflFeren- 

 tiation of its walls which are formed by the production of cells 

 from the syncytial mass. These cells give rise to successive 

 muscular rings, each of which is composed of two transversely 

 striped muscle cells, each cell forming one of the lateral halves 



' It was probably this similarity which led Weisraann to think that the heart 

 of the fly was formed by the hollowing of a solid muscular cord. 



