232 BULLETIN OF THE 



along tne ventral border of the gland, as in the other embryos, under- 

 goes a change of structure at about the level of the second nephrostome. 

 The lumen thex'e begins to enlarge, and the wall to become thinner. 

 Farther forward, the cavity of the tube becomes greatly dilated, and 

 the bounding wall is reduced to a delicate pavement epithelium, having 

 the same appearance as the peritoneum covering the pronephros. The 

 tube again contracts shortly before attaining its most anterior bend. A 

 similar dilation also occurs in the following stage, in the description of 

 which I shall again refer to the chamber thus produced and suggest its 

 possible function. 



The pronephros at this stage is completely invested in a loosely fitting 

 capsular membrane. The cells of which this envelope is composed have 

 become very thin, so that they form a delicate sheet not more than 6 /x 

 thick. The nuclei occur in slightly enlarged portions of the cells. They 

 are rather small, and show a tendency to be flattened in the plane of 

 the layer. At the lower outer angle of the myotome, the capsular 

 membrane is continuous with the myotomal sheath, as in the earlier 

 stage. The capsule covers the pronephros so loosely as to leave exten- 

 sive spaces between the enveloping membrane and the tubules. These 

 spaces, together with those between the convoluted tubes, form an ex- 

 tensive and complicated system of sinuses, which bound the pronephric 

 tubules on every side (compare Fig. 28, belonging to the next older stage). 

 Behind the last nephrostome, a considerable space intervenes between the 

 wall of the capsule and that of the duct. There is thus formed a single 

 continuous but irregular channel, which accompanies the duct through- 

 out its entire course ; it is also prolonged into the region of the gland as 

 a large ventral sinus, which is triangular in cross section (compare the 

 lower of the spaces marked sn. sng. in Fig. 28). This channel is the 

 fundament of the posterior cardinal vein. The course of the vessel may 

 be traced at this stage for a short distance behind the point where the 

 ducts open into the cloaca. There are two veins connected with the an- 

 terior end of the pronephros. Of these the ventral one is the larger, and 

 is continuous postei*iorly with the cardinal vein. The dorsal vessel of 

 the pronephros also unites with the cardinal vein by means of the spaces 

 between the tubules. After passing forward and leaving the pronephros, 

 the ventral vessel proceeds medianwards to empty into the sinus venosus, 

 this vessel constituting the ductus Cuvieri, The dorsal vessel of the 

 pronephros can be traced forward into the head. In the somite next in 

 front of the first nephrostome, it lies between the ganglion nodosum and 

 the myotome. It can be traced for some distance along the base of the 



