HALL: MESONEPIIROS AND MULLERIAN DUCT IN AMPHIBIA. 59 



somites of the older larvae, I think a more significant series of averages 

 is found by using for the six anterior somites only the ten younger 

 larvae (columns B-K). 



I should not include the average for somite 19 {^.^), as I do not 

 think it trustworthy, from the fact that the mesonephros curves venti'ad 

 in that somite and hence is cut more or less frontally, which renders the 

 determination of the number of units difficult. It is this curving which 

 sometimes causes the units to appear as if they extended posterior to 

 the opening of the duct into the cloaca, as in larva P. of the diagram. 



In discussing the significance of these averages, I will first call atten- 

 tion to the diagram of larva A, which shows a single fundament for each 

 somite, from the ninth to the eighteenth at least. It will be remem- 

 bered that, while in posterior somites these fundaments could not be 

 identified in the continuous blastema formed from the fused mesomers, 

 in anterior somites they remained distinct and became the "hlastulae of 

 the first order," between which there soon appeared the smaller " blas- 

 tulae of the second order." In the anterior four or five somites, usually 

 not more than one of these smaller blastulae appeared between each two 

 of the first order. Candad, they appeared in increasing numbers. From 

 the last column of the table (p, 57) it seems that the blastula of the 

 second order is generally absent from somite 9, sometimes present in 

 somite 10, and typically present in somites 11-13. In somite. 13 a 

 second one is sometimes added, making the total number of blastulae 

 two to three. This process continues, an extra blastula being generally 

 added in every second somite, as represented in the last column of the 

 table (p. 57). 



Primary and Secondary Units in AmUystoma and IcMhyoj)his. 



It will be instructive to compare the condition in Amblystoma with 

 that found in Ichthyophis by Semon ('92). In Ichthyophis tlie tubules 

 of the first set are strictly segmental in position and resemble in structure 

 the primary units in Amblystoma. Soon there appears a second set ; 

 these are first seen as small intersegmental balls of cells (blastulae), rest- 

 ing on short outgrowths from the duct. Then appear in succession a 

 third, a fourth, and a fifth sot, all in line (horizontally) with those of the 

 first set, and all opening into the duct. Semon suggests that we have 

 here a hint as to the origin of the dysmetamerism found in the higher 

 Amphibia and in the Amniota, where, he suggests, the multiplication of 

 the fundaments may take place very early. He assumes that in Ich- 

 thyophis these second units arise by a budding from those of the first 



