142 W. r. R. WELDON. 



vertebrae. Fig. 21, the most anterior, passes through a verte- 

 bral region, and shows the segmental vesicle, with its lumen; 

 i the section passes through the attachment to the peritoneum 

 (which in the vertebral regions is becoming smaller), but not 

 through the connection with the protovertebra. The next 

 section (fig. 22), through the commencement of the interverte- 

 bral region, shows the solid cell mass, with a few cells (w. d.) 

 split off from its outer portion. These cells are the rudiment 

 of the Wolffian duct. In the next protovertebral region this 

 cord ceases to be visible. Fig. 23 shows a section through the 

 commencement of the next protovertebra, passing through the 

 solid wall of the corresponding vesicle, which has no trace of 

 the duct. 



These cords of cells are present at this stage in four inter- 

 vertebral areas, behind protovertebrae five to eight inclusive. 



With the formation of the thirteenth protovertebra the solid 

 rudiment of the Wolffian duct becomes more distinctly split off 

 in the intervertebral regions, while opposite the protovertebrae 

 it appears as a solid appendage of the wall of the segmental 

 vesicles, with which it is perfectly continuous. 



At the same time it extends backwards into the ninth inter- 

 vertebral region. 



In an embryo with fourteen protovertebrae there are eight 

 segmental vesicles with a lumen opposite the protovertebrae 

 five to twelve inclusive. All these have the Wolffian duct as a 

 solid knob on their outer wall, while in the corresponding inter- 

 vertebral regions there appears a distinct lumen in the duct, 

 which is more or less completely split off from the rest of the 

 intermediate cell mass. 



The relations of the duct and vesicle in an embryo with 

 fourteen somites are shown in fig. 24, from the second segmental 

 vesicle of such an embryo. In this figure the segmental vesicle 

 (s.v.) is seen to have a large lumen, and the solid Wolffian 

 duct {w. d.) appears attached to its outer wall. 



In fig. 25, from the next intervertebral region behind fig. 24, 

 the Wolffian duct has a large lumen, and is attached to the 

 solid intervertebral cell mass. 



