216 BULLETIN OF THE 



has appeared in the organ. It is then found that the lumen occupies 

 the position of the major axis of the ellipse, and that the nuclei of the 

 bounding cells lie close to the interior surface of the wall. If a tube so 

 constituted be compressed laterally, so that the lumen wholly disap- 

 pears, it is evident that the cells of the opposed walls would be likely to 

 accommodate themselves to one another so as to present an alternate 

 arrangement of their nuclei. 



Opposite the middle of a somite, the relations are somewhat different. 

 Here the two layers of what I shall hereafter call the pronephric pouch 

 do not remain confluent at its dorsal extremity, but separate, the outer 

 becoming continuous with the somatic layer of the protovertebra. the 

 inner with the deepest layer of the thickening, and thus finally with the 

 lateral somatopleure. In this region the body cavity can be seen to 

 project for a short distance between the two layers of the pi'onephric 

 pouch, as shown in Plate I. Fig. 6, ccel. This figure demonstrates very 

 clearly the relations of the pouch to the lateral mesoderm and the over- 

 lying somites. 



In the case of the younger set of embi'yos which have been con- 

 sidei'ed in this stage, it will be remembered that the boundary between 

 the lateral mesoderm and the protovertebi-a^ was evident only in inter- 

 segmental regions. In the somewhat older individual now under con- 

 sidei'ation, the constrictions between these two portions of mesoderm have 

 advanced into segmental regions as well ; so that now, for the first time, 

 the precise relations between the fundaments of the nephrostomes and 

 the protovertebrse lying above thein can be accurately determined. The 

 last remnant of the communication between the protovertebral cavity 

 and the body cavity I shall call the communicating canal, following in 

 this the nomenclature of Renson ('83), The section shown in Figure 6 

 passes through this canal {can. comn.). and it is to be especially noted 

 that the constriction between the somites and the lateral plates takes 

 place dorsal to the region of communication between the pronephric sys- 

 tem and the body cavity. Immediately dorsal to the pronephros, the 

 somite sends out a lateral fold of the somatic layer, which is destined to 

 form the capsule of the pronephros, to which I shall have occasion to 

 refer in later stages. 



In somite IV., the division of the pronephric mass into a dorsal and 

 ventral part is faintly indicated, but the dorsal part shows no trace of 

 the lumen which is destined to become the third nephrostome. In this 

 embryo, the constrictions between the protovertebrse and lateral plate 

 have advanced into more posterior regions. In somite V. the constric- 



