MUSEUM OF COMPARATIVE ZOOLOGY. 261 



series of sections backward, however, the chamber enlarges greatly, even 

 before the nephrostomes are reached, and is separated from the ventral 

 portion of the body cavity only by the lung bud. Between the first and 

 second nephrostomes, the pronephric chamber is divided into two parts by 

 the fusion of the distal edge of the glomus with the somatic peritoneum 

 covering the pronephros. Still farther posteriorly, an open communication 

 is established, not merely between these two portions of the pronephric 

 chamber, but also between the latter and the general body cavity. 



In almost all the larvae of this stage, the mesonephric tubules have 

 appeared, and in many individuals they have already opened into the 

 duct. There is always a space intervening between the pronephros and 

 the mesonephros, in which no tubules are developed. This interval ap- 

 pears to be subject to some variation, but in the majority of cases it 

 comprises four somites. 



In the most anterior region of the mesonephros the tubules show traces 

 of a metameric arrangement, but this is wholly lost in more posterior 

 regions. These relations can perhaps be best illustrated by the accom- 

 panying table, which shows the positions of the right mesonephric tubules 

 in the larva, whose pronephros is represented in Figure 64. The somites 

 have been reckoned by reference to the spinal ganglia, but the results are 

 here expressed in terms of the original metamerism of the myotomes. 



Somite III. — Pronephric nephrostome I. 

 IV.— " " II. 



" V. — Tubules absent. 



VI.— " 

 VII.— " 

 VIII.— « 

 IX. — 1 mesonephric tubule. 



tubules. 



k 



a. 



Each tubule of the mesonephros (Plate VII. Fig. 53) has the ordinary 

 form, which has induced several authors to call it "sickle-shaped," and 

 consists of cells which are wholly devoid of yolk spherules, in which the 

 nucleus occupies almost the entire body of the cell. Along the region 

 which corresponds to the cutting edge of the sickle, a few loose cells (fnd. 



