The pronephros of Chrysemys margioata. 25 



of the lateral mesoderm, or with the lower end of tlie nephrotome, 

 occurs in the region of segment X at all events. 



A comparison of the condition of the pronephros in embryos C, 

 D and E proves that the posterior pronephric tubules develop after 

 the duct and not at the same time — thus differing from the con- 

 dition found in the development of the ant. pronephric rudiment. 

 This "arrested" development of the posterior part of the pronephros 

 is important in relation to the question of the backward extent of 

 the pronephros and the meaning of the pronephric duct. 



The later pronephros extends over segments VII, VIII, IX and 

 X: it is this portion of the pronephric rudiment alone which 

 becomes functional — the rudiment in segments III, IV, V and VI 

 atrophying at an early date. From the start the kidney in segs. IX 

 and X is affected by a rotation lateralwards of the duct — so that 

 the latter becomes laterally situated, and a consequent increase in 

 length of the nephrotome region occurs — whereby the tubules in these 

 segments come to open into the coelom in a region closely adjoining 

 the connection of the nephrotome and the lateral plate, and even 

 (in seg, X) into the enlarged lower end of the nephrotome. 



The backward growth and future connection of duct with gut 

 will be dealt with separately. 



Hence we must regard the early development of the "Sammelgang" 

 of the pronephros, and of the pronephric duct behind, as absolutely un- 

 broken, and speak of the whole structure as pronephric : we must con- 

 sider the duct (from seg. X to cloaca) as formed in the same way 

 as the "Sammelgang'' region, viz. segs. VII, VIII, IX and X: but 

 the former region has in addition to contend with the modified 

 environmental conditions, which are in existence at the time of 

 the arrested development of the part of the pronephric duct behind 

 the pronephros. The following table gives the chief results in 

 tabular form. 



This table shows clearly: 



1. The lagging behind of the early development on the right 

 side as compared with that on the left side. 



2. The gradual atrophy of traces of the primitive kidney groove 

 in segments II, III, IV, V and VI: this degeneration is not a very 

 regular process, and may happen earlier or later in individual 

 embryos. 



The irregularity in the backward extent of the pronephros 

 in each embryo on each side may be quite easily due to observa- 



