698 EMILY RAY GREGORY, 



other; through the remainder of the body they are closed off from 

 the coelom and have a glomerulus, fed by branches from the aorta, 

 dependent from the dorsal wall of the median bubble-like end; they 

 early become S-shaped, but increase in number, irregularity and size 

 of cells with development. 



Metanephros. 



The first indications of the coming metanephros are found in embryos 

 of stages XI and XII. The Platypeltis embryo XII (4) [Plate 48 

 Figs. 63—68] shows the earliest stage of the branching of the ureter 

 from the dorsal wall of the duct at its opening into the cloaca. Here 

 the evagination of the dorsal wall at first shows only a less rounded 

 contour, which is hardly noticeable and one would not remark the 

 more pointed outline in the following sections except for what is seen 

 in later stages. The small size of the blastema of denser staining 

 cells just outside the wall of the evagination is very striking. Con- 

 sidered by itself one would almost certainly interpret it as arising from 

 the division of the cells of the wall or to as due the stimulus (mechanical 

 or chemical) of the cells pushing against it. The reduced and irregular 

 blastema of the mesonephric tubules lies on the median side of the 

 developing ureter. In Platypeltis embryo XII (2), if we follow a few 

 sections beyond the first point of the opening of the Wolffian duct 

 into the cloaca, we see a sudden outgrowth in the dorsal wall push- 

 ing directly up into a small but compact blastema of denser staining, 

 concentrically arranged cells, lying just above the duct. Here, too, a 

 careful study will separate the blastema of the metanephros from that 

 of the mesonephric tubules, which is in contact with it. Following 

 the blastema to the anterior end we find several cells in mitotic 

 stages at a marked distance from the mesonephric rudiment, which is 

 clearly not the centre of growth for the new blastema. 



Plate 48 shows the condition in an embryo of stage XI (Figs. 57 

 to 60) and two sections also from one of stage XIV (Figs. 61 and 62). 

 A most careful study of sagittal series of this and later stages gives 

 similar results. As the ureter pushes forward and the blastema around 

 it becom^s larger, the end of the mesonephric blastema is for the 

 space of two or perhaps three sections so closely connected with it 

 that from such older stages it would be impossible to determine a 

 separate origin for the metanephric blastema. From such series and 

 from a more cursory glance at those just described, I was for months 

 in doubt as to the true condition. The lack of sufficient reason for 



