Geo. C. Price 133 



and a small though varialtle luimlier of the closed tubules as well. 

 At all times the anterior end of the excretory 'systeui is a little back of 

 the posterior end of the branchial system. For quite a long period tlie 

 history of the differentiation of the pronephros of the adult is simply 

 a history of the crowding together of the tubules, as will appear from 

 the following account of the organ in a series of embryos of different 

 ages. Before beginning, it may be stated that in all cases every section 

 of the parts described was drawn, different colors being used for the 

 different tubules. This was highly advantageous in all cases, and was 

 absolutely necessary in the older and more complicated specimens. The 

 arrangement is not usually exactly the same on the two sides, but unless 

 there is some really important difference only one side will be described. 



The first embryo of the series is a little older than the oldest of stage 

 A, and the process of crowding together of the tubules has just fairly 

 begun. The first tubule is in the sixteenth segment and is not con- 

 r.ected with the rest of the svstem. In segments sixteen to twenty-four, 



Fig. 27. Sagittal section ttirougli tlie auterioi- end of the excretory organs in au 



embryo in which the tubules are beginning to be crowded together, formed by the 

 combination of two adjoining sections. There are here five tubules in the space of 

 three segments. (J, segmental duct ; t, tubules. 



that is in nine segments, there are twelve tubules. Back of this the 

 position of the tubules has not been affected. If all were arranged seg- 

 mentally the first tubule would be in the thirteenth segment. The first 

 three or four tubules are not so well developed, as the following, and look 

 as if they might be in the process of atrophy, although this is not at all 

 certain. Fig. 27 is a combination drawing made from two sections, 

 showing five tubules in the three segments, twenty to twenty-two. There 

 is quite a little space as yet between the tubules. The segmental duct 

 is almost without a lumen, but a little further back the lumen begins, 

 and continues witli only a few interruptions to the posterior end of the 

 duct. However, the duct does not open to the exterior. 



In the next embryo the first tubule is in the twenty-secoiid segment, 

 and in the six segments, twenty-two to twenty-seven, there are fifteen 

 tubules. Thev are not evenly distributed, being more crowded in the 

 anterior part than in the posterior. But in no case are two tubules in 

 actual contact with each other. One or two at the anterior end are 

 quite rudimentary, and there was some doubt as to whether they actually 



