58 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



(2) Are any primary units added in any somite after the first unit in 

 that somite has appeared] 



(3) What is the typical number of primary units for each somite 1 



It will be seen that in somites 9, 10, and 11 there are more units in 

 the younger than in the older larvae. In attempting to account for 

 this fact, two possibilities must be considered; either some of the units 

 degenerate or they migrate caudad, thus shortening the mesonephros. 

 In order to determine which of these alternatives is the true one, a line 

 was drawn (Table 1) separating the ten larvae 1 7 mm. or less in length 

 {B-K) from the ten which were over 17 mm. in length (^L-U),^ and 

 the average number of units in each of the six anterior somites on one 

 side of the line (B-K) compared with the average of the corresponding 

 somites on tlie other side (L-U). The averages for somites 9, 10, and 

 11 of tlie younger ten (0.5, 1.4, 2.2) are greater than those of the older 

 ten (0, 0.3, 1.5). The averages of somites 12, 13, and 14, on the con- 

 trar}^, are in general less for the younger than for the older ten (2.3, 2.4, 

 3.5, as compared with 2.6, 3.1, 3.4). The decrease in the number of 

 units in the anterior somites as the larva grows older is thus probably 

 due entirely to a shortening of the mesonephros. The preponderance 

 of the units in the older ten, in somites 12, 13, and 14, over those in 

 the younger ten, does not, however, quite make up for the paucity in 

 somites 9, 10, and 11 of the older ten, and the conclusion is natural that 

 the shortening of the mesonephros has also affected the region of somites 

 12, 13, and 14, and that on investigation it would be found that there 

 had been a backward movement of units from this region into the somites 

 immediately behind. Pursuing the method employed above, I divided the 

 ten older larvae into two lots. On counting the units in somites 15, 16, 

 and 17 of each lot, I found the sum of the units in the younger five to 

 be si.\ty-six, that in the older five, seventy-two ; that is, there is a pre- 

 ponderance of 0.4 of a unit per somite, for somites 15, 16, and 17 in the 

 oilier five over those in the younger. As this preponderance is not large 

 enough to suggest an actual addition of new units in each somite, it is 

 confirmatory proof of a shortening of the mesonephros. 



In answering question 1, question 2 has also been answered in the 

 negative, at least for all but the one or two most j)Osterior somites. 

 There remains the question : What is tlie typical number of units for 

 each somite] The simplest way to determine this would seem to be 

 to find the average for each somite in tlie twenty larvae. As the 

 arrangement of the units has been much modified in the more anterior 



1 Larva A of the diagram is not included in the table. 



